Was thinking the OP, but for anyone; have you looked for another DIYer in your area? A chance to pool resources and ideas always seems to help. Even stopping by a vape shop and see if someone can give a class. I noticed a friend's DIY gear and it gave me ideas. A good vape shop would probably know someone that would do it on the side since shops are now rather limited in what they can do in the way of teaching.
As to measuring, I go ML measurements. For example, I figure everything in percents that total 100. Then, to make a 5ml sample, I just divide all the percents by 20.
At first, I didn't get so critically specific as to base percents. I made a 100ml batch of PG/VG knowing that my nic and flavoring were also in PG. I did 70ml of VG and 30ml of PG, knowing that when I added the nic and flavoring, the overall PG would increase and the final product was about 50/50. I have changed a little with practice and experience, but, I still don't get to caught up with exact PG and VG levels. I figure I am with a percent or three on my final PG and VG ratio.
Also, having the right kind of gear helps. I have one glass measuring cylinder that measures to 50ml. I use that for making my base. All other measurements are by syringes. I have a syringes that measure to the 10th of a ml that I use a lot. Seeing as I make small 5ml test samples, it gets used a lot. I also have a 3ml, 5ml, 10ml and 20ml syringe. Since most of the 5ml sample will be PG/VG base I use the 5ml syringe for that. If nic and flavoring will total to 20%, then 80% of the 5ml bottle will be base. Dividing 80 by 20 gets 4, so the base content will be 4%. I put that in first. It got me started and familiar with using syringes.
As to finding good mixes; I have a LOT of little 5ml bottles. I just got on Amazon and bought a lot of bottles of several sizes. Price wise, they were cheaper, but they also came from the Far East and took a little longer. I got 5ml, 10ml, 20ml, 30ml, and 50ml.
Another thing was notes. I just happen to use a spreadsheet for everything. Inventory, recipes, notes, etc. I can't say how much that has helped me. Can also use Word files. My spreadsheet has links so I can jump around pretty quickly.
As to exact steeping, nah, I just steep them all the same. Pretty much 2-3 weeks. I just didn't want it to be an issue when I was evaluating a mix. I think, at the time, I had favorite juices that I had bought. I just made sure that I had about a month's worth and vaped that while making/steeping samples. Their is a bit of patience needed.
I kept things simpler at first, but I have grown a bit in DIY. You will, too.
Anyway, hope helps.