I take Utrogestan too. The 100mgs dose is more for women who take it for 25 days a month ( more or less continuously.) BUT there are some deviations from this- see what other people have said.
I am on 200mgs for 10-11 days every 4-6 weeks, whereas I was also allowed 100mgs for anywhere from 15-21 days but this hasn't suited me as I got bleeding at odd times. This is being prescribed privately and I would be able to access a scan if there were any problems.
The reason for taking it at night is that it can cause drowsiness and dizziness in some people- so not safe if you drive etc.
I don't think it would be wise to cut the capsules in half- the progesterone is in an oil suspension and it would be messy and quite hard to get the correct amount as you might spill some or not cut in exactly in half- you can get 100mg tablets but that's not what you've been prescribed.
The thing is, you won't always know if you need a higher dose- the dose helps control the amount of thickening of the lining, so unless you have a scan, or you have symptoms like odd bleeding which might indicate a thickened lining, the it's hard to know if you need more.
Others will be along with their experiences and advice.