my trt journey begins!

[20YO]

Hi guys, quick background for ya: I'm a 20 y/o from Australia.
Previous 12 months, I've had a reduction in libido, sexual function, big change in body composition (less muscle, more lower abdo fat despite good diet and training), chronic lethargy and a reduced sense of wellbeing.

Over the space of 3 months, I had numerous blood tests done; my worst 3 results were 16.6, 12.5 and 12.1 nmol/L or about 350-450 ng/dl total test. lh, fsh, estradiol, thyroid homones all came back normal.

After a couple failed attempts, I've finally found a doctor who recognises a) my levels are low and b) that I'm symptomatic!
Consequently, he's prescribed me 100g of 10% testosterone cream (compounding pharmacy). I will be picking this up in the coming days.
It was a huge relief to finally find a doc who knew what was up, and who wasn't afraid to help!

I asked him about IM test; he said to see how i go with cream first. I also asked him about HCG , SERMS, AIs; again he said to see how I go. My plan is to apply the cream daily for a month, then return for bloods... discuss how my symptoms are etc... The prospect of HCG and the like is quite appealing, because I want to keep some nad size as well as remain fertile... perhaps if I push the fertility issue he will prescribe in future...

The doc said to use roughly as much gel as I would tooth paste, and use my libido as a guide for upping/downing dose haha
Are there any more precise ways to measure it?
If i applied say 4 grams a week, that'd be 400mg testosterone - would that be sufficient?
How might absorption compare to IM? And are there preferred body areas/times of day to apply?
Also... roughlty how long til you start 'feeling it?'

I'll also be getting on the vit d, DAA, maca, fish oil to try and maintain some sort of endogenous production...
Anyway, feel free to chime in with your advice, experiences! I'm looking forward to it!