In a (very condensed) nutshell, estimates are based on credits or time. Personally I find credits easier. A normal FT student on a degree will do 120 credits in an academic year and have a STULOAD of 100.0. Your apprentices are then a percentage of that, eg if they’re doing 60 credits STULOAD will be 050.0.
STULOAD is always based on what is studied within the period 1st Aug – 31st July, so non-standard years get a little trickier but as long as you remember the standard academic year it gets easier.