Greetings 2+2!

[kayaker]

The best advice I can give you right now is to not chase a 3-flush or 3-straight. If you're not relatively comfortable you have the best hand on the flop - or a good draw to the best hand - don't chase. If you can see the turn for free, that will help you, but don't waste money trying to runner-runner someone.

If you really want to calculate your odds for RR, it goes like this:
((outs/cards left) x (outs/river cards left)) x 100 = your percent to hit

Outs = the total number of cards you can hit to continue. On the river this will decrease. If you hit the T on the turn, you have (4x)Ks and (4x)8s for outs. If you hit K or 8 on the turn, you have the (4x)Ts for outs.

In this case, that's 10 outs on the turn and 9 on the river for the flush. Plus 9 outs on the turn and either 8 or 4 (6 or 3 if the turn helps the flush) on the river for the straight.

Best case scenario (Th on the turn):
(19/47 x 15/46) x 100 = 13.2%

Worst case scenario (K or 8, non-heart on the turn):
(19/47 x 4/46) x 100 = 3.5%