Tuning Question

I tune my rig, a masthead sloop (Seafarer 26) using The Complete Riggers Apprentice as a guide. I have no problem tuning the sidestays:
10-12% on the lowers
15-20% on the uppers

The guide then states:
"Make the forestay about as tight as the intermediates, and the jibstay and backstay about as tight as the uppers."

Shouldn't these numbers vary depending on the relative length of the back and forestays? I cannot achieve the desired tension in both at the same time. Any ideas?