I have a topspec measuring cup and they will send one out to you if you ring them. It's easy to feed the right amount in that a 600kg horse will need 600g a day etc but it is expensive so worth getting the measurement right I think!
I normally take some of each feed home, weigh it in terms of the measure I will be using (so 1 scoop = Xg) then can work it out in my head from there on!
Their measures, full to the top (but level, if you understand what I mean LOL) are 300g, so a 600Kg horse needs TWO of them.
I have used a standard coffee mug (as a 300g measure) but I'd rather use the real thing TBH, all Top Spec suppliers should have them on their counters.