HB and FB motions are scripted off the snap, aside from in pass blocking and running routes for catching. That means on anything but a pass play the snap reaction matters exactly 0. If your HB is a scatback or primarily used for pass blocking it is worth investing 35, up to 55 even, but unless he is specializing for one of those 2 things specifically it is not worth investing in at all.
And if you want them to pass block or catch passes, that's fine - even with low snap reaction they can do these fine because of positional advantage - so it's not something you invest in if you are just dabbling in catching passes or pass blocking.