I think the reason why you run out of mana so quickly is because you maxed ES before TK. I personally would have done it the other way round.
Note that on page 1 of this thread renegademaster mentions 3 possible builds: 1 ES + 1 TK, 1 ES + 20 TK, and 20 ES + 20 TK (prebuffing not taken into account). You appear to be working towards the latter setup, but your current (intermediate) state is not recommended in the guide, and I assume the mana issue is exactly the reason. With 20 ES + 1 TK you deflect a maximum amount of damage at the highest possible mana cost. Unless you have a really big mana pool thanks to good gear, this setup will leave you quite vulnerable.
Try getting hold of a Gerkes Sanctuary shield, this will provide the damage reduction to your energy shield.
FO and ES buils is really for Duelling, as there lots of cold immunes out there that cannot be broken even with Infinity Runeword (it has a conviction aura).
Either try a duel element build as SirPoopsAllot Suggests, or go Pure Lightning and get an Infinity polearm for your merc. Whilst Inifinity is expensive the build is very powerful and can go anywhere, and excellent at magic finding.
It depends. My ES sorc (http://forum.diii.net/showthread.php?t=727081) had a pre-buffed lvl17 ES and 20 points in TK, so I cannot speak from experience. But you can do the calculation: if a monster does ~100 damage in Hell, and you have a lvl 40 ES but only 1 point in TK, then it will cost you approx. 184 mana (0.95*100*(2.0-0.0625)). Taking into account that you might get hit by 5 gloams at the same time, you're already talking ~1000 mana just to survive being in the wrong place at the wrong time. Then you want to have something left for casting spells... well, you can make your own estimation.
I was quite comfortable with the ~2000 mana I had, but that was with 20 points in TK and a lower lvl ES. A monster doing 100 dam would only cost me 54 mana (and 28 life, most of which was cancelled by PDR/MDR gear).
Question on balance (I bookmarked good links back in 2008 in case I'd ever need them, but alas I deleted them; search function isn't bringing up good results):
I know people spend hours figuring out a good life/mana balance with ES, so if any of you out there can answer, it'll be great. In general (if that's even possible), what life/mana ratio am I looking for:
I actually sort of agree with Kurumi here also because of the way you can choose to alter your life and mana via charms.
I myself made a full block fully synergized ES sorc with base 0 ES and can prebuff to about 20-25 which absorbs most of the incoming damage that isn't blocked. I only have 900 base life and 1100 base mana which is enough for most things but I am going to be remaking the sorc to actually put hard points into ES so that I can hit the 40 cap on ES or rather closer to it.
I'm also going to rework the sorc to have much lower base life and much higher base mana as well as changing around significant portions of gear (example I upped my perfect 35/13 viper because this sorc uses Stormshield/Monarch and my new one will not). I'll let you guys know how the new sorc turns out but I do think that while I will lose out on some damage the increase in mana and therefore overall survivability will be incredible. As it is I can take several Nihilthak CE's and Gloam volley's before dying but I want to try it with the giant mana pool I had on an even older ES Sorc (that one was Orb rather than Fire which is what I choose to make now).
If you have decent vita charms I don't really think you need hard points in vita which stands to reason there is no balance required. The stats I listed above are without a CTA on my Sorc (I just restarted and only have a 2 BO CTA on my Hammerdin). With a reasonable CTA I would hit about 700-800 life after BO which is more than enough to tank what little damage actually gets through with the new build I am planning. In fact I am fairly sure the current iteration of my Sorc only has about 100ish vita which is only 200 life. Far less than what even a 1 BO CTA would provide me.