Couple of thoughts.

1). The derailuer capacity as you have calculated assumes that you will be running small ring to small cog and large ring to large cog (cross-chaining). I don't usually worry about the chain going slightly slack on small chian-ring cross-chaining becasue you ought not be doing that anyway. The opposite, large ring to large cog is also bad, but I make sure that it will at least get there so the rear derailuer does not break if I forget.

2). Front derailuer capacities, especially newer ones, are highly suspect. The newer ones have bent cages to work with specific sizes of chainrings in each location. These suck if you want to re-gear. The BEST front derailuer I have ever used is a cheap mid-70's Falcon. If you have a bike co-op near by, you can probably get 10 for $10. Regardless, you are going to have to try what ever you have before you know if it is going to work. Again, chain rubbing on the bottom of the cage in small ring to small cog cross chaining is not something I really care about.

3). As someone mentioned, you are limited to 24T on the front. Shimano now has a 12-36T 9-speed cluster, but you are in 8-speed land. You might be able to build your own if you are technically savvy enough, or switch to 9-speed (need new shifters). Some have noted trouble getting their Shimano derailluers to swallow the 36T cog. again, you will not know if you need to swap until you try it. Regardless, if you cannot push 24:34 up a hill, it would be faster walking.

4). I HATE Mega range, prefering a more consistant ratio between gears, but that is your call. I noted that you are spending alot of time at the bottom end of your cassette (the little cogs). I might suggest that you might go with a larger middle ring to keep your avaerage more towards the middle of the cassette. It will wear better and give you a bit more selection up or down as you dial in you cadence.