Well, this is what I did. It's probably not 100% accurate and there may be an easier way of working it out, but it will do for now. It might sound complicated but once you get the hang of it, it's not that tricky.

First you multiply the current pool of $36000 by 0.85 (what the TAB gives back) and you get $30600. Then divide that $30600 by the current dividend of $5. That gives you $6120. That's roughly how much has been bet on the horse so far.

Now you multiply the new pool of $37000 by 0.85 and you get $31450. Divide that by the new amount bet on the horse ($7120) and you get the new dividend after your bet. $4.41 or $4.40.