Proofing on a heating pad

Lately I've been doing the final rise for sourdough bread
on a Sunbeam therapeutic heating pad. By my measurements,
it's too hot, but the results so far have been excellent.
I'm wondering if others have tried it, and to what effect.

The recipe is:

200 g whole wheat flour
200 g white bread flour
210 g hot (steaming) water
blend flour and water to lumpy mass
cover and keep warm (100 F) 30 min.
add 200 g starter (100% hydration)
mix till smooth
add ~15 g salt
mix at least till bowl cleans up
cover and keep warm (100 F) 30 min
stretch and fold
cover and keep warm (100 F) 39 min
stretch, fold and form loaf
place in covered, oiled Corningware dish

Place Corningware directly on heating pad 
with temp probe between dish and pad, on low.
Cover pad and dish with inverted cardboard box.

refresh starter and place on pad under box 

Set timer to 5.5 hours, give or take. I use the
activity of the starter as a gauge of progress.
The temp probe starts out around 115 F or so,
very slowly rising to an indicated ~135 F by bake
time. The starter has to be stirred down at least
twice to control overflow, volume gain 4-5X. 

The room temp has been around 60-62 F for the batches
so far. Presumably the temp will rise as the season
advances.

By all I've read, 135 is too hot, but the yeast seems
to do just fine. I've checked the thermometer (Taylor
remote-probe oven thermometer/timer) against boiling
water and ice, it's off by no more than 2 F. Raising
the bread and temp probe on a cooling rack lowers the
indicated temperature about 10 F and reduces blow.  

Bake is 30 min covered at 350 F, 15 min more uncovered.
Nothing special, oven preheated ~5 min.

The starter is a mongrel. It came from Bario Bread in
Tucson AZ but lost activity when I tried incubating it
during the past summer at ambient (80 F) temp. A gram 
or so of commercial (Red Star) yeast revived it and it's 
not been tampered with for at least six months, or about
24 refresh cycles. It never was particularly sour and
hasn't changed to my nose or palate. The starter, on its
own, smells faintly like latex paint.

I anticipate that as the summer progresses the room temp
and so the proof temp will continue to rise and so a bit 
concerned about how hot is too hot. 

Thanks for reading, and any thoughts.

bob prohaska