Dave,
glad you got it sorted ,
measuring to the top of SB4 instead of top edge of the SB3 gives a difference of 25mm, coupled with the 17 degree backwards angle of the seat back area would cause you losing the 8mm.
at least it was worth double checking everything before you started grinding off welds cos of the half a dergree prob.
yeah at times after time away from the build the mind can play tricks on you
I sometimes look at something that i,ve done a 100 times & i end up with a blank expression
Glad you,re sorted..............now get that chassis finished
cheers
andy