For my first one, I went for ~17% which is now 24+ months old. The most recent one (7+ months old) is about 20% (1 part grain: 4 parts
Santalum album).
I warmed
each maceration only 3- 4 times for 2-3 minutes in an espresso cup - say once every 1-2 weeks (don't remember exactly) for the first few weeks. This is just to 'kickstart'/quicken the process at the starts. The water temperature was just beyond touch, not too high. I didn't want to lose any highly volatile smell due to excessive heat. After that I left them as they are. Now I simply take them out every week and shake them for few seconds. If you're making a large batch I guess you can use an electric stirrer, but I'd avoid heating (unless you're careful enough).
Pre-maceration: Abdullah (Mellifluence) told me to remove all the skin type innards from the musk grains before putting them in sandalwood as they can have adverse effect. He said hair in maceration is safe (which can also appear with the grains). But I didn't risk it and removed those too. Sometimes those innards will retain a lot of semi-dried grains. So use a non-soaking surface to work on and use some stainless steel forceps (or something similar) to separate them. Be wary, grains are incredibly stinky, pissy - almost headache inducing. Whilst in UK, when I prepared grains for the 1st time (separating and macerating), it made our whole house stinky. Upstairs, where I worked on smelt quite pissy for few hours. It was quite windy and cold at the time. So I couldn't opened the windows as long as I wanted to. I could smell it from downstairs. My olfaction was highly curtailed for few hours.
You can use those innards, pod shell, and hair in tinctures. With those the tincture is lot more pissy than just grain tincture.
As for how long you want to wait and the proportion, it depends on the length of time it is allowed to macerate, how strong you prefer it to be or their uses in compositions. Best if you make a few ones with different proportions and compare them for your purposes, bro. I made them mainly to layer with perfumes and ouds and to make my experimental and weird attars. It is fun.
