Bottling - Carbing Sugar question

So I've been unhappy with my past few brews carbonation levels. I've using those corn sugar packets that come with kits -4.5 or 5 oz. My beers have been under carbed, and I think I know the reason.

I use the frozen water bottles/ice bath method to keep fermentation temps down, but once the fermentation is substantially complete, I stop and the beer rises to about 80 or so.

Now the chart here says, the warmer the beer is the more sugar you use.
http://www.howtobrew.com/section1/chapter11-4.html

Is this the temperature the beer is when bottling, or the warmest the beer ever got after fermentation completed?

For example, my double ipa, ferment at 66-68, while dry hopped it reached 80, finally a cool spell in new england and now i'ts back down to 70 in my basement. for carbing, my beer temp will be 70, but I believe I should look at the chart for 80 since the beer reached that point, released the c02, and there was no more c02 production so the beer wouldnt have absorbed any more.

So I bought a real scale, and I'm going to use the proper amount of sugar, hopefully this fixes my problem as I cant afford to get into a kegging/kegerator setup.

Also is there is an online calculator for this? I'm thinking volume of beer, temp of beer, desired carbonation level = amount of corn sugar?

The best treatment of this issue I've found is John Palmer's How to Brew. The chart you reference works really well. I adjust the result for the size of my batch (anywhere from 3 to 6 gallons depending on what I got out of the brew kettle).

I single stage ferment my beer for 2-4 weeks so fermentation is completed before bottling (lower gravity bitters and milds are generally done sooner than imperial ipas and belgians is my rule of thumb).

The temperature of the beer is the temp when you bottle it. I just tape a thermometer strip to my carboy for a few minutes while I gather my bottling gear.