Increasing a synthesist's eidolon hit points?

[Coinshot Colton]

Since a synthesist's eidolon can't get feats, does that mean that the only way to increase its hp is by evolution pool and increasing its Constitution every five levels? Was playing around with a build, and only managed to get around 50 hp by level 7 (biped build). Is that okay...? Seems pretty low for a tank, since its Con score starts pretty low.

[Taku Ooka Nin]

I think the Gnome gets the favored class bonus of +1hp to the eidolon per summoner class level.
Also, keep in mind that the HP for the sythesist is total equal to
Eidolon's HP + summoner's HP before it despawns if you use Fused Link to keep it alive.

[Coinshot Colton]

That was stupid of me. Completely missed that bit about being able to scarifice the summoner's hp. Thanks!!

[Taku Ooka Nin]

Yeah, so just go with Gnome, have your Con be 20, get your Cha as high as you need it to cast all of your spells without items, and then take Toughness.
(1st) 16+8 {24} ([8+5+3]hp+[6+1+1]temp)
Each additional summoner HD beyond 3 is
(4th) 11 (5+5+1)
Each Eidolon HD is the same as the first unless you buff constitution
(1st) 8 (6+1+1) + any extra con bonuses you give.

So your total summoner HP by level 7 should be
(7th) 86+54 {140} ([8+6+6+6+6+6+6+35+7] + [6+6+6+6+6+6+12+6])

So, there you go, your theoretical max for a level 7 summoner without using any kind of cheese, without items, and without evolutions.
You could give yourself a +2 or so to Con to increase the eidolon's HP by 6 (up to 60) but you already have a stupidly high amount of HP for level 7 at your combined 140 hp.
By level 7 you can take the ability Score mod twice, meaning that you could have the eidolon have 66 hp at summoner level 7, hence increasing your total HP to 152, so,
just to recap, at level 7 you could have more HP than a level 20 caster who takes the extra HP for favored class, AND has a constitution score of 14 or 15.

Do it, totally do it.