Be prepared to buy some VG/PG and nic base in small amounts for trial, say 36 mg/ml (me 100mg), a liter of VG and 4oz PG. They'll all be useful. Mix flavorings separately and your nic base according to your target, e.g. 12mg vs 18mg. One thing about DIY is it will give you the convenient opportunity to start bringing the nic down after 3 years. Trust me, you don't need it. Although I enjoy a bit after 3 years myself. Nic base is easily targeted by proration. For example 2 parts PG or VG to 1 part 36mg nic will bet you a 12mg target or at 5:6, 15mg. To avoid confusion, 18 mg/ml is equivalent to the designation 1.8% nic typically noted on labelled juices.
Start slowly at graduating downwards on nic level if you're really stable at 1.8%. Folks sometimes experience a kick back when dropping too suddenly. You may have resistant thresholds too. Consider targets of .2-.3 downwards. Stay there a week or more or until comfortable there. You can keep some standby juice in another device for urges at your old or higher nic level (if find you stepped down too suddenly).
Started experiencing a sensitivity to PG a few weeks in. Isolated that it wasn't erratic coil/device performance (dry burns, shorts) or nic level itself. Didn't immediately suspect PG. Took some time, more than a year, to slowly get used to higher PG levels in juices. Why I've pretty much been DIY from the jump and modifying purchased juices. Still some irritation these days if level or use goes high but impressed with what I can now handle. I usually try to mix max VG recipes wherever possible and find very little basis of the criticisms for doing so. Some of my very best results have been 100% VG yet also the most complex recipe at 70/30 VG/PG. Flavorings are mostly at least part PG these days so I'm not avoiding the richness of nuance for concerns about PG. It's just not essential to a good flavorful vape or production. Consequently, I use exclusively VG based nic knowing there will always be PG component. The juice calculator above allows you to punch in the relative % nic used as well as designate whether a flavoring is PG or VG based. So you'll have a fairly tight handle on the proportion.
Pick a simple flavor profile you really like and are familiar with like a fruit, say strawberry, and a basic recipe. At least enough nic to do several 30ml variations. Start with smaller batches and use the 100 drop method (1 drop=1%). You can split 30's up into separate batches if you get a comparison idea. I often have several batch samples running on different devices. Put all those clearos to use!
A few ideas. Hope these help.
Good luck K.