Why in heck are you concerned with low end on Webra .50? Let 'er rip!
You made me curious enough to dig out a Webra from my trove. First one I came to is a .91, but it has the Dynamix carb which I think you have by the 'slide carb' note. A CCW tweak of the idle knob pulls the low end needle out from its seat,, as does opening the venturi. Appears to increase fuel flow as with most other engines, those with air-bleed carbs excepted. Method of low end adjustment I generally use to get a good starting point is to pinch the fuel line at low throttle and note if if speeds up or sags and dies. If it speeds up, it was on the rich side, if it sags and dies it was likely too lean. Iterate on this a few times with a 1/8 - 1/4 turn or so of the low end needle knob (you move the needle via a short linkage with the Dynamix) and you'll soon have it set to an optimal idle mix. Then check acceleration, as you may have to set it richer or leaner from best idle setting to get a good transition. FWIW, my Webras, all the Austrian variety vs the ones made in Germany, have seemed to like the McCoy MC-9 plugs best.
Abel