Welcome to the wonderful(and often very expensive)world of telescope making!,I have not seen the sidewalk plans ,but nails to secure mirror in the cell sounds like a bad idea,a couple of plywood discs with a bolt running through center of each with a "tee nut" on one side of each disc,back of mirror is secured to one side of one of the discs with globs of silicone,drill 3-equalateral holes in the other plywood disc and install tee nuts and thumb screws(which of coarse serves to collimate the primary mirror)a bolt runs up through the center of the pair of wood discs which will be tightened after collimation,see Richard barrys book "build your own telescope" for a more detailed description of this cell design.
I try to locate the focal plane somewhere between the racked in and racked out point of the focuser to accomadate a range of eyepiece focal lengths(usually 4mm to 30mm).
I have made most of my mirrors so i can only guess as to accuracy of advertised F.L. a master optician with a sphereometer can achieve 0.001",an amateur may be off 2" or 3" or more!
,if you make a mistake in locating focuser and your using a cardboard tube, save the disc that was cut out and use this to plug the old hole,fill around edge with putty and paint over.
Good luck.