Is 100 mg nicotine level standard?
No. you can order base in many different strengths.
For a beginner, you may want to start with a small 60ml bottle of, say, 24mg per Milliliter (this means each ml will contain 24mg of nicotine). Three milliliters will contain (3x24) 72mg. Each 10 milliliters will contain 240 mg of nicotine (10x24). The entire 60ml bottle will contain (60x24mg) 1440 mg nicotine. So, a little 60ml bottle of 24mg/ml will make 240ml of eliquid if you are reducing strength to 6mg/ml. And that's a lot of eliquid!
You will need additional PG or VG for dilution (about 180ml). You might want to use VG/distilled water mix instead of PG or a customized mix of PG/VG or just PG. To generalize, if you add 1ml of 24mg/ml to 3mls of plain PG, you will have 4mls of a 6mg/ml solution. (1ml + 3ml = 4ml and 24mg + 0 = 24 /4ml = 6mg/ml). If you add flavorings, then it will put the concentration off by what you add. If it's just for yourself it's probably not enough you'll really notice for your first try. Let's say you add 1/2ml of flavoring to the 4mls. 24 / 4.5 = 5.34mg/ml. But if it's for someone else you will need more accurate calculations and measuring to get to an exact 6mg/ml. You will need measuring instruments, such as pipettes or syringes that measure up to 3, 5 and 10ml, and bottles to mix into and a pen and paper for notes, and I use a serving tray as a work base so if there are any spills they are contained.
Nicotine base can be stored in the freezer but do it in a way kids can't get to it (another reason to buy small bottles of base) and no kids or pets around when you are mixing. Learn to write down every step every time in case you get distracted - and it happens more often than not (the kids just ran thru the house while you were mixing - you're trying to watch them but now you've forgotten - did you add 3mls or 4 mls? ) Now you have an outline, it's still good advice to do a lot of additional reading in the DIY threads before you get started. There you'll read opinions of what flavoring company is considered the best for xyz flavoring. While another company makes a better abc flavoring. Hope this helps - have fun and stay safe !