Aluminium bending compensation.

I need to bend some 3mm thick 5251 aluminium through 90 degrees.

The plan is to clamp it to a steel former and wipe a roller around the
outside in order to form the curve.

Given a 3mm thickness and no deformation, the inside radius will be
17mm and the outside radius should be 20mm.

I'm using a" k factor" of 0.5 as the radius of the bend is more than
three times the thickness of the material.

Plugging these values into a couple of online bending compensation
calculators gives me a bending compensation length of 29mm.

This I read to be the effective length of flat material I need in
order to get the curve I require.

Now the part which is confusing me.

The length of the 90 degree arc is 1/4 of the circumferance of a
circle

So for the inside 17mm radius, it is 0.25 x 2 x 17 x 3.141 =  26.7mm.
And for the outside 20mm radius, it is 0.25 x 2 x 20 x 3.141 = 31.41mm

This makes 29mm sound a reasonable length.

However, the calculators also give me a "bend deduction", which is
explained as the amount the material will stretch, of 10.9mm.

Does this mean I need to remove 10.9mm from the 29mm in order to
compensate for the stretch ?

It sounds unlikely to me that the material will stretch by 33% over
the course of the bend.

Adrian