thats the reason the service manual says to be carefull when unscrewing the lowers from the uppers...
the rod can be damaged...
I think this simple test at least will give you an idea
and in reality its the fork springs and eventual coil binding that will limit your max travel, thats the reason people put stiffer springs, "top out" springs like 88-90xr600 forks have or preload spacers, again preload spacers is more of a cheap fix for initial travel in regards to fork dive when braking hard, not because you dont have enough slider length
look at your bike and measure without the boots and see what you get lengthwise, Ill bet its more than 12 inches
actual fork travel on our bikes is 11.8 inches...less when taking into account sag and all