-----
cold-setting the fork blades to achieve a 100mm opening is perfectly fine
you need to make sure the fork is in good alignment by checking with a fork alignment gauge
all framebuilders and serious cycle shops have this instrument
if you were to just begin pulling without any way of checking you might pull one blade 5mm for example and the other not at all
if you have access to a bicycle co-op in your area they may have the required tool
-----