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
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The bend deduction sounds like the full difference between a square (2 * 20mm) and a radiused (29mm) corner. Bend and measure a narrow test strip.
jsw
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