I'm confused by the dosages. Is it calculated on body weight?
No.
Insulin doses are worked out by trial and error.
Find patterns. Overnight patterns, before and after meal patterns (preferably with a carb count), patterns around exercise.
In your case, you were on insulin already, so there is already a rough idea on how much insulin you need. (To give you an idea, some people only need 15 units in total a day, although those usually have T1 and are thin, others need over 300 units a day.)
Currently, you're on 58 units of insulin so there is a starting point from which your nurse is working, it doesn't make sense to expect you'll need less than that on a different regime.
She has said start the Tresiba once a day, 50 units, then fixed dose of 15 units of Trurapi before meals. She said if I have a carb free meal, leave it.
When I saw the the Dietician straight after she said count my carbs (am happy to do) then calculate 1 unit of Trurapi to 5g of carbs.
I know you can't advise as such, but by these calculations the DN seems to think I consume 75g of carbs per meal? I certainly don't.
I agree with your worries on overdosing here. It may well be that the DN assumes some 75g of carbs per meal if she's thinking eatwell plate, I like the dieticians thinking better.
The Dietician also talked to me about correction dose where my bg is higher than target before a meal. I get that, but didn't explain how many units per mmol.
Yes, I know I need to speak to them to clarify, just looking for anyone who might clarify some of it for me.
No one knows what your correction factor is at the moment, it's trial and error. Your CGM will be very helpful to find patterns.
In general, it isn't advised to do corrections until 4 hours after eating because you'll still have active insulin on board.
She said I can take the Tresiba am or pm, which is best?
With Tresiba, the longest acting of all long acting insulins it really doesn't matter, take it at a time that suits you!
I take it before bed, no matter if that's 10:30 pm or 5:30 am, it's a very forgiving insulin when it comes to timing.
Thanks for any input, wish me luck.
Much luck coming your way, I really hope that the freedom of a basal and bolus regime will make you as happy as it does me once you get the hang of it!