I offer knowledge from an engineer who has observed the issues of working with polymer clay for many years.
1) Oven thermometers built into ovens (regular ovens, toaster ovens, convection ovens, any oven...) are calibrated to keep an oven 'around' the temperature you set it for. Not exactly AT the temperature you set. Not even all that close for much of the time in the oven. And the internal thermometers in an oven are also usually not well calibrated. Which brings us to purchasing another thermometer to put inside your oven to see what's going on in there.
2) Almost all oven thermometers purchased off the shelf are as badly calibrated as internal oven thermometers. The best way to accommodate for this is to buy 2 thermometers, -from DIFFERENT manufacturers- and believe the oven is at about the mean temperature between them. For instance, if one reads 150F and the other reads 100F, the oven spends more time around 125F in general.
3) If you REALLY want to know what is going on in your oven, buy something like a Thermoworks ChefMate
Yes, expensive. OH so worth it. (And when you use it with your actual food cooking you'll be amazed.) But back to polymer clay - Use an oven temperature probe, which measures the temperature of the air in the oven. To the 10th of a degree kind of thing. Within something like every second. It has an alarm you can set for both the highest and lowest temperatures that are acceptable for what you're doing at the moment.
I have a fairly good convection oven (not the top of the line, but up there). I was shocked to discover when I set my oven for 275F, the oven air temperature varies from about 190F to 295F multiple times within just minutes, without even opening the door. Just during a normal closed-door cooking cycle. So the idea of 'KEEPING your oven at 275F' or whatever is just not going to happen. The best thing you can do is try to keep it as close as possible, and keep it in there a long time, hoping it will get to the soaking temperature (275F) that it needs for long enough.
Just wanted to offer some information from a knowledgeable and researched standpoint.