Invocation of Nehek

OK, so I'm planning on playing a small game Friday night and wanted to get this right. Lets say I have two casters, one of which has the master of the dead rule. 

Invocation allows units to REGAIN wounds. Master of the dead allows units of skeletons to increase beyond their starting size. 

So, in this example I'll keep the numbers simple. 2 Casters both with Invocation and one of these has master of the dead rule, (I) and (IM) plus 20 Skeletons.

Turn 1. 20 Skeletons take 5 wounds (15) (I) casts and the unit regains 5 (20) (IM) casts and increases the unit by 5 (25) happy so far? Good, let us continue. 

Turn 2. Now (I) cannot have any effect on the unit as it's not taken wounds so nothing to regain but (IM) can cast and increase the unit size, again by 5 (30) 

Turn 3. The skeletons take 5 wounds (25) And now here's my question. Can (I) cast attempting to regain the 5 lost wounds as at the start of the turn the skeletons were 30 in number OR can he not regain those wounds as the unit is over it's initial starting strength at the start of the game (20) ? 

I'm happy that (IM) can still add to the unit at this stage but the difference could mean having so 30 or 35 skeletons to face. 

Thanks in advance 

Greg