Goosie, I would definitely use 100mg vaginally and see how you get on with it. If you get a good bleed with that dose, then you will know it is enough but you might want to have a scan (preferably after a period) to check that you are not taking too little or too much.
I think the licensed dose is high because Utrogestan is only licenced to be taken orally in the UK - for no reason whatsoever. Therefore, because about 50% of it gets lost in your digestive system, the licensed dose has to be high but you can take much less if you use it vaginally.
Professor Studd would not be prescribing women 100mg of Utrogestan for 7 days each month if it was highly risky or dangerous. I have proved that this is definitely enough for me by having a scan immediately after a period and having a lining measurement of 2mm and I take quite a high dose of oestrogen. I would argue that had I taken any more Utrogestan, it could have caused me problems because the lining was already thin enough. I realise that I am only one person and that everyone is very different but if it this works for me, chances are it will work for other women.
If you turn the whole thing on its head, it would be impossible for all women to need exactly the same amount of Utrogestan to get the same results. Therefore, it is trial and error and the dose must be adjusted to suit the individual and yet again, once size does not fit all.
If you are progesterone intolerant, you have to be realistic about how much Utrogestan you can take and if you set the bar too high, you will end up cheating like I did and ditching out after a few days - incidentally, it was still enough to give me a proper period.