In Daniels Running Formula 1st Edition he had a table which suggested a slowdown of 1sec/mile/deg above 16degC. He did not include this in current 2nd edition of book but that would back up the calculator above you quoted which says 13sec/mile difference over 16-29degC temperature rise.
I ran in the past in Carribean & hottest part of south eastern turkey (>40degC) and the heat definetly takes it out of you but it'll stand to you. Just make sure you don't get dehydrated as I left myself drift a bit too close to the edge in the past and its not a good idea - you seem to be doing good so far though.
you will be trying to avoid the carribean and trukey this summer i take it! be a nice break in november all the same!!
how is you training going? any good sessions completed lately? how is your tempo pace looking these days? i recokon for myself i need to be at least as low as 6:06m/m to be in with a shot of 2:50 marathon, differrent for other people though.