If the head angle is 69 degrees with a fork with an a2c of 515mm, will it be 67 degrees with a head angle of 545mm?

also what angle will the seat tube be – 72 degrees with a fork of 515mm, will it slacken it to 70 degrees?

rather than just a yes/no answer – anyone care to explain to me how i work this out for my self please?

Head angle and seat angle will slacken by the same amount.

70deg seat angles will make you do wheelies unless you are under 5′

so what is the answer re fork length?

A rule of thumb is a 10mm change in fork length gives a half degree change in the head angle, so your change in axle to crown would give a slackening of 1.5 degrees.

For increase accuracy you need to brush up on your trigonometry (SOHCAHTOA).

to work it out think triangle, you know the fork length, you need to know the distance to the rear dropout, from the front hub and from the fork crown and work it out from there.

Yes, you use trigonometry to work it out

so let me get this right – unsagged my HA will be 67.5 and seat angle 70.5?

Sagged at 25% head angle would then change to 69.5 and seat tube to 72.5?