Maybe give it something like this? *Just an idea*
Make it a % of BONUS health shield so it doesn't lose effectiveness and it should be everytime you go back to base.
like 2%/3.5%/5% of bonus health shield. So if I get a Warmogs, and have three points into Runic Blessing everytime I recall I get a 40 health shield. Sure it's not impressive, but that's why its in the tank tree - for tanks it'd be better than that 50 health shield crap, and you _should_ get it everytime you recall.
Where as if I get a Warmogs, Randuins, Righteous Glory my bonus health shield will be: 95 (still not impressive but it's not supposed to be ridiculously game changing)
Or if you want better, maybe 1/2/3% of maximum health?