@John *******
you have a 60 day multiple entry tourist visa that is valid for 6 months but you can only stay a maximum of 60 days at a time ,you can leave Thailand as many times as you want in the 6 months that the visa is valid for and get 60 days on returning to Thailand so no need to do a 30 day extension .
I had this visa when I came in April this year ,you can also get 60days on the last day your visa is valid for to get 8 months from the visa,you can then either get a 30 day extension or do a boarder run and get a 60 day exemptions then extend it by 30 days giving you 9 months in total.
If you are going to get another 60 days just before the tourist visa runs out I wouldn't leave it to the last day I would do a boarder run about 1 week before the tourist visa runs out.