Dropping my 2 cents here.

This may not be true for 2+ node Cluster, but I always try to put more RAM on server than the total amount of SQL instances I have if, hypothetical scenario, I need to run those on one node only.

For instance:

RAM on Server A: X GB

RAM on Server B: Y GB

So... RAM Instance C + RAM Instance D < X + Y

That's in order to avoid the situation that two instances have more allocated RAM than actual server's RAM.