Building the ExtraNG - how do I know if my CG is even close before the maiden?

If the plane only balances with the battery pushed way forward, it means your current nose weight is insufficient to bring the center of gravity (CG) back to the planned point. This is super common when using a generic motor; motors vary wildly in weight even if they have similar kV ratings. Don't worry about the lead weight unless the balance is wildly off. A small amount of weight, maybe just some carefully placed pellets or lead tape near the nose, is almost always better than risking a poorly trimmed plane that has to constantly correct itself in the air. Since the plans don't specify the exact battery placement, they are relying on the pilot to find the sweet spot, but you should keep the battery near the planned mark if posible for flight characteristics. If the weight issue is small, moving the battery slightly back might be enough before you add any weight at all.

Regarding the control throws, if you're worried about damaging the foam, dialing down the radio is the smartest move for the first few flights. Those huge 45-degree swings are likely overkill for a foam glider, even an aggressive one like the ExtraNG. While high expo helps make the control response feel sensitive and 'floaty' at low throttle, excessive throw can cause the plane to twitch or oscillate if the throttle is increased quickly. Try reducing the throw percentage in your radio settings to maybe 50-60% of the full throw, and then use the expo curve to fine-tune the feel. You can always increase the throw later if you want a more abrupt feel.

When I built mine, I initially had a lighter motor than anticipated, so I ended up shifting the battery back about a quarter of the length of the Velcro strip to hit the marks. That small adjustment was enough. The only thing that mattered was having the CG within the allowed tolerance range, not necessarily being right on the line. For the throws, I started with them dialed back just in case, even though the plans suggest a high throw, and slowly built up the deflection as I got more flight time in her.

I had a similar struggle getting the CG right on a foam glider, but mine was a size up. Even when I finally got it balanced, the rudder kept wanting to yaw hard left during the slow climbs. I spent ages trimming the elevator up a tiny bit to keep the nose stable in the air. Sometimes getting the balance point is only half the battle; you gotta fine-tune the control surfaces til it just feels right in the wind.

One thing I'd add about balancing on fingertips with a pack that can slide on the Velcro, the reading shifts every time you pick the plane up, so you're chasing a moving target. What worked for me was setting the battery where I actually wanted it, marking the fuse sides with a pencil where it tipped level, then checking that mark against the plans before deciding wheather any nose ballast was even needed. On the throws, check the horn position too, since moving the pushrod in one hole on the control horn changes the effective deflection a lot more than most people expect, and it saves you radio fiddling entirely.

Honestly, don't worry too much about being perfectly on the line; within 2% of the mark is totally fine for a foam rig. When I built my last one, I found that if the battery placement was off by more than 50mm from the drawn mark, the handling felt wierd regardless of the flight. It’s more about consistency than precision. I ended up adding two small lead weights right inside the wing root area to fine-tune the balance, wich was way easier than trying to move the entire power pack around. That small tweak fixed everything for me.

You gotta remember those plans are just a starting point, not gospel. I learned that lesson the hard way, trying to acheive perfect precision with these foams. Before everthing got so digital, you just built it up until it felt right in your hands.

As for those huge control swings, before you even mess with the radio settings, check the hardware. Sometimes the whole problem is the servo or the linkage itself being too tight or binding up. A slight rub point on a pivot can make a big difference in how aggressive the flight feels. Old school, you just checked the whole setup for smoothness. I remember once having a plane that handled fine until it started twitching, and it wasn't the throw; it was just a little friction in the aileron horn assembly.

Update: turned out the pack placement was the whole issue. Took the tip about marking the fuse sides once it tipped level, then taped the pack down at that spot. Balance sits basially where the drawing shows now, no lead needed. Left the big throws as drawn and just dialed the expo way up, feels much saner on the bench. Maiden planned for the weekend.