I always look up U_K's ECL document (version 5) for this sort of thing.
ECL +0.44/level of Integrated Sorcerer or Wizard spells
ECL +0.38/level of Integrated Cleric spells (includes bonus Domain spells)
ECL +0.28/level of Integrated Druid spells
ECL +0.15/level of Integrated Bard spells
CR = 2/3 ECL. Some other forum members have called this the "KR" instead of CR, to clarify matters.
Adding 20 integrated cleric levels would add 5 KR by this method. Balors are KR 22 already by this system... which would make the total KR 27.
For a party of four ECL 19 characters, a KR 27 fight would be an EL+1 fight, which is classified as a "Very Difficult", party victory 25%, utilizing all party resources for the fight, good for a Nemesis. It wouldn't hurt to make it slightly more deadly either, as the "Very Difficult" category covers EL+1 to +4. (Note: Equal EL means it's a 50/50 fight, could go either way. Positive ELs mean the party is actually at a power disadvantage. Equal CRs or KRs, on the other hand, are weighted in the parties favor.)
To raise the EL to +2, you'd have to have a KR 32 Balor. To do this by adding spellcasting levels alone would require 40 integrated cleric levels. Of course, integrated levels should never exceed HD. So instead, I'd reccomend integrating 20 levels of cleric, and then adding 10 LEVELS (HD included) of cleric on top of that... if you also added 10 levels of wealth. Without adding equipment, you'd need 15 levels of cleric to make the Balor EL+2.
EL+1 is probably fine though. It's still a nasty encounter that will scare the living daylights out of the party and give them more than a run for their money. Making it EL+2 is still a POSSIBLE fight to win, but the players will only win by the skin of their teeth, careful and clever tactics, maybe a bit of well optimized character builds thrown in for good measure, and other such things.