The mathematical progression isn't bad, but I don't like the idea that the tombheart can have fewer HD than a warden. I'm also not sure that we need to go into so much detail, though I'm ok with it. I'd like a basic tombheart to start at about 14HD, in any case. They should be more powerful than the wardens, I think.
I would prefer them to start out with the same HD as a Tomb Warden.
How about "A Tombheart has Hit Dice equal to the highest Hit Dice Tomb Warden it is linked to, plus 1 HD for every 5 Hit dice of the Tomb Wardens it is linked to. e.g. the Tombheart of four 11-HD Tomb Wardens has a base of 11 HD, plus 8 HD (44 HD divided by 5), or 19 Hit Dice."
The Tombheart size could go Medium 11-20 HD, Large 21-40 HD, Huge 41-80 HD
etc.
That would translate to:
One 11 HD Warden (11HD/5=2HD) -> 13 HD Medium Tombheart
Two 11 HD Wardens (22HD/5=4HD) -> 15 HD Medium Tombheart
Three 11 HD Wardens (33HD/5=6HD) -> 17 HD Medium Tombheart
Four 11 HD Wardens (44HD/5=8HD) -> 19 HD Medium Tombheart
Five 11 HD Wardens (55HD/5=11HD) -> 22 HD Large Tombheart
Six 11 HD Wardens (66HD/5=13HD) -> 24 HD Large Tombheart
Seven 11 HD Wardens (77HD/5=15HD) -> 26 HD Large Tombheart
Eight 11 HD Wardens (88HD/5=17HD) -> 28 HD Large Tombheart
Nine 11 HD Wardens (99HD/5=19HD) -> 30 HD Large Tombheart
Ten 11 HD Wardens (110HD/5=22HD) -> 33 HD Large Tombheart
Eleven 11 HD Wardens (121HD/5=24HD) -> 35 HD Large Tombheart
Twelve 11 HD Wardens (132HD/5=26HD) -> 37 HD Large Tombheart
Thirteen 11 HD Wardens (143/5=28HD) -> 39 HD Large Tombheart
Fourteen 11 HD Wardens (154HD/5=30HD) -> 41 HD Huge Tombheart
...
Thirty Two 11 HD Wardens (352HD/5=70HD) -> 81 HD Gargantuan Tombheart
While for Large Tomb Wardens...
One 16 HD Warden (16HD/5=3HD) -> 19 HD Medium Tombheart
Two 16 HD Wardens (32HD/5=6HD) -> 22 HD Large Tombheart
Three 16 HD Wardens (48HD/5=9HD) -> 25 HD Large Tombheart
Four 16 HD Wardens (64HD/5=12HD) -> 28 HD Large Tombheart
Five 16 HD Wardens (80HD/5=16HD) -> 32 HD Large Tombheart
Six 16 HD Wardens (96HD/5=19HD) -> 35 HD Large Tombheart
Seven 16 HD Wardens (112HD/5=22HD) -> 38 HD Large Tombheart
Eight 16 HD Wardens (128HD/5=25HD) -> 41 HD Huge Tombheart
Nine 16 HD Wardens (144HD/5=28HD) -> 44 HD Huge Tombheart
Ten 16 HD Wardens (160HD/5=32HD) -> 48 HD Huge Tombheart
...
Twenty One 16 HD Wardens (336HD/5=67HD) -> 83 HD Gargantuan Gargantuan Tombheart
Hmm...
No, I think we should drop this approach and simplify the whole thing.
How about the following.
A Tombheart must be linked to at least two Tomb Wardens, which must all have the same Hit Dice. The Tombheart has a size and Hit Dice according to the following table:
'No. of Tomb'''''Tombheart
''Wardens'''''Size''''''Hit Dice
'''''2'''''Warden -1''As Warden * 1
''''3-4''''As Warden''As Warden * 1.5
''''5-6''''As Warden''As Warden * 2
''''7-8''''Warden +1''As Warden * 2.5
''''9-12'''Warden +1''As Warden * 3
'''13-16'''Warden +2''As Warden * 3.5
'''17-24'''Warden +2''As Warden * 4
'''25-32'''Warden +3''As Warden * 4.5
'''33-40'''Warden +3''As Warden * 5
Example: A wall containing Seven Large 16 HD Tomb Wardens will have a Tombheart one size larger with 2.5 times as many Hit Dice, for a Huge 40 HD Tombheart.
That way we don't need to worry about calculations, just present a table that gives reasonable Hit Dice values.