The bad news:
You're probably not going to get what you're looking for especially with tolerance. Certainly not on half a tablet anyway. As you're obviously already aware: they're in mcg and not mg and that's total per tablet (unlike the patches, for instance, that actually contain mg in total but the drug is delivered in mcg per hour).
I can tell you this though:
It's no myth by any stretch of the imagination that benzodiazepines dangerously potentiate the effect of Fentanyl and exponentially so. So you may want to think about this. And bearing in mind that Valium has a long half life i.e. anywhere between 20 and 100 hours. And that's just to eliminate HALF. And the metabolites takes longer.
Fentanyl on the other hand is tricky. Depends on where you look. But its short half life is debatable. Anywhere between 3 and 12 hours. Again: that's just to eliminate HALF. And again: the metabolite takes longer.
Studies have also shown that Fentanyl can be redistributed in the body before being metabolized. This is one of the reasons why users have died of overdoses a little while later in spite of being brought back from the brink of disaster using Naloxone. It has a short duration of action (not to be confused with half life) and therein lies another danger i.e. repeat dosing because the action has worn off but it's still floating around. Easy to repeat doses for effect not realizing that concentrations are climbing. And then nod-boom-bang!
Recent data being released (UK and Scotland anyway) also shows that a very high proportion of supposed Fentanyl related deaths are not due to Fentanyl on its own. I'm by no means playing down the dangers. But seems to me there's a fair amount of fear mongering going on in the media when it comes to Fentanyl. I know that's not good harm reduction information at all and probably better safe than sorry (better to be fearful of it). But I'm also tired of being flimflammed by the media in general.
The above being said: obviously not a good idea to use with other opioids and for obvious reasons.
Anyways. How you proceed is up to you of course. From what I gather they're orally disintegrating tablets. Saliva does destroy Fentanyl though so I cannot tell you what the bio availability is of these tablets (but I'm sure the manufacturers would have taken this into account and allowed for it). Not knowing what else is in them: not sure as I'd go trying to dissolve them and IV (I'm sure that's crossed you mind no?).
As most around these parts will tell you: you can take more of anything but you can never take less. Also: Fentanyl has a nasty habit of apparently doing nothing at first and then all of a sudden hitting you in the face like a sledge hammer and without warning. Point being and in summary: don't use those tablets until you're sure there's nothing else floating around in your bloodstream and start slow, wait a while, bearing all of the above half life information in mind, use another one.
Or you could just toss them of course (although experience around these parts has taught me that's usually never an option on the list).
Here's a fairly decent document that shows the half lives of many of the usual suspects (your Fentora tablets are even mentioned). Some of the figures may surprise some very much.
I wouldn't call them definitive nor worst case scenario (the figures I mean) as these figures do vary from individual to individual (and not to mention from website to website but these at least come from a laboratory).
Fentora:
www.nps.org.au
Full disclosure and worth noting: I'm not a user. The above based purely on research (save for the occasional test with regard to the concomitant use of benzodiazepines, z-drugs, and Fentanyl for research purposes). I make this clear only because active users may have more pertinent information and input on the topic. I'm just presenting the science as I see fit (with a sprinkle of opinion).
