BR515N and adding a cabinet questions

[JeepSahara]

I just bought a BR515N combo (which I love) that I use to play small gigs with. That setup is more than enough for those settings. However I recently found out (after my purchase) that I have a couple outdoor gigs coming up in the fall to play for. So I was thinking about adding a separate bass cabinet to the combo for these. I am specifically looking at the BR410N-8. I know I need to match 8 ohms to 8 ohms which give me 4 ohms well within the limit of the BX500 head. However I am wondering if the BX500 has enough watts to make the 410 cabinet and the 115 in the combo for an outdoor gig be loud enough. Is there a way to calculate (if I run these parallel from the back of the amp) what wattage each speaker in the 410 cab and the 115 in the combo would be getting and would that be enough while not straining or be on the verge of hurting the BX500 head. I am not in the know of watts/ohms and the like I just show up an try not to suck to bad. I am totally in the "what i don't have in talent I make up for with volume" club. Thanks in advance.

[MyMusic]

Try this. Carvin.com :: BR115N-8

[JeepSahara]

While I think adding a 15" cabinet wouldn't be a bad idea I just was thinking (and could be wrong) that since the combo I have has a 15" in it that I could get a nice well rounded sound by adding the 410 cabinet. Plus since that head comes out of the combo I would be able to use any combo of the 115, the 410 and the BX 500 head. That is if the head will push all of that when it is all being used at the same time. I like the idea of being change my setup around since I play bluegrass, irish pub/folk music, church stuff and popular songs in the different groups that I play in. So any thoughts on the 410 cab with my current combo?