The A10C only does a simple Distance/Time = Speed calculation to give you an average speed IIRC, in which case yes you'd have to do as described e.g. if you need to be at a target in 30mins (0.5 of an hour) and you're 150nm away, it'll tell you to fly at 150/0.5 = 300kts. If that's too slow, you have to work something out yourself.
You don't need to do that in the Hornet though, because you can enter a required GS into the UFC when entering TOT data and it'll compensate for you. e.g. same scenario as above but you require 500kts at the target, it'll direct you to fly slower than 300kts at first, so you can fly faster later on and arrive at the same time.