Originally Posted by GrayJay

Change in fork length has the effect of changing the head tube angle "HTA" (and also changes the STA and BB height). It takes a fork length change of around 20mm to produce 1 degree of change in the HTA. If you want to determine the rake change needed, then you need to know what the new effective HTA is after the longer fork is installed. Then, with the HTA and the desired trail value you can back-calculate the rake needed to get there. A change of +/- 3 mm is probably not going to be perceptible, I wouldn't bother with re-raking it you find your trail is anywhere within the range of 54-60mm.

You can accurately measure the HTA with a phone app, but you can also skip measuring the HTA and fork rake and just measure the fork trail directly using a long strait-edge, a 4' drywall square or a long carpentry level works well for this. Line it up along axis of the heat tube to mark where the head tube axis hits the ground, then measure distance back to a plumb line to ground from the axle centerline. I would definitely get a measure of your existing trail before you go bending the fork.