Old thread this, but I'll try ....
I only read this today, I've make a lot of madras/vindaloo BIR style cuury.
I'd been racking my brains for ages trying to get the correct sour taste.. I'd tried lemon juice, lime juice, cider vinigar, tamaring puree; non gave the correct flavour.
Before reading this (honest) I tried a few tbs of the juice from tinned plum tomartos, and found it gave pretty much exactly the flavour I was trying to achieve!!
So you can immagine how happy I was to have this coroborated by an independant curry fiend!
In my opinion your 3 or tbs of pureed tinned toms is spot on!!!