Can some please tell me what are the odds of me making a backdoor flush, if I have 2 suited cards in my hand?

Also, what if I flop a backdoor flush + backdoor straight draw. What are my odds of completing one of them?

Thanks

You need two cards of your suit to come on consecutive cards. You have seen 5 cards, leaving 47 left in the deck. 3 of the seen cards are your suit, leaving 10 of your suit unseen in the deck of 47.

On the turn you need a 10/47 event to occur and if that happens on the river you need a 9/46 event to occur. To find their combined probability we can multiply them.

10/47 * 9/46 is your first answer.

Straights are trickier than flushes (for backdoor draws) because the turn card impacts the number of outs for the river card. If the turn card leaves you with a gutshot, you only have 4 outs. If it leaves you with an open-ended draw you have 8 outs. If you compound this fact with the fact that the turn card could work towards both the straight and the flush draw giving you many outs on the river, you see that this second question is trickier to answer in general (i.e., it is trickier to answer for all backdoor straight and flush draws; but straightforward to answer for a given specific hand).

So I have ~4% chance of making a backdoor flush?

Is that 4% large enough to alter any of my decisions to continue with some hands. For example could it be more profitable for me to bluff in certain sports where I will win an extra 4% of the time?

Of course 4% can be large enough to change a decision you make. Sklansky has talked about this in at least one book (but probably numerous books). The one I can remember for sure is Poker, Gaming, and Life book. An essay called "A Note on Extra Outs" or something like that. Adding a backdoor draw to a hand with a pair can be quite valuable.

Actually, now that I stop to take a look, it seems that same essay is in Sklansky's Theory of Poker book (word for word at a quick glance). Small Stakes Hold'em (p. 101) has a section specifically discussing backdoor outs. They suggest that a backdoor draw is worth approximately 1 additional out. So of course adding an extra out can swing a decision.

To be honest, from both your questions posed so far it is pretty obvious you are new to poker (or at least new to thinking about poker mathematically). I would seriously recommend books like Small Stakes Hold'em (it is for limit poker, but very instructive about these basics concepts and question), the first Harrington on Hold'em Book (it is for tournament poker but again, it is extremely instructive about the basics), and then Professional No Limit Hold'em.

You should be able to calculate things like "the probability of making a backdoor draw" with ease to be successful as a poker player. Forking over the money for those books and spending the time and effort to read them will make you a lot more money than they will cost you for sure.