First thing to do is search here in the Forum... we've discussed this at length before.
For good measure, the best rule of thumb is to use the 60/20/20 rule and fine tune from there. AJ McClane prescribes proportioning the leader in three basic sections:
so that 60% is butt, 20% is taper, 20% tippet
On the back end, the diameter for good transfer from fly line is a consideration. For the front, presentation of fly. The transition from butt to tippet should consider avoiding radical step-downs for dry line leaders.
For short stout sinktip leaders, three sections suffice. 4-6' 15# winter steelhead for instance, or striped bass fishing on a sinking line. I extend the tippet for short leaders to keep the knot away from the working end, in other words fudge the 20% rule on the tippet end.
For longer leaders
, the butt section should be split into 60%/40% itself. The taper into two 50% sections or thirds, depending on the step-down desired to the tippet. The tippet can be either one-pc or split 50% with a step down as well.
To keep things simple, I use Maxima spools by pound which automatically provides a reasonable step-down in diameter for you. I burn a lot of leader in a year but the tippet is where most is cycled, taper next, butt section often lasts a third to a half of a season if a durable line like ultragreen maxima is used.
For example, for stripers on an intermediate clear line I use a 7' 6" foot leader with 15# flouro tippet and a 40# butt. (all left column numbers are maxima pound test ratings except for flouro tippet, the bottom most item):
40 - 30" (60% of 60% of 7 feet = ~30")
30 - 20" (40% of 60% of 7' = ~20")
25 - 10" (half of 20% of 7' = ~9", round to 10")
20 - 10" (same)
15 - 20" I usually extend to 24" to provide a fly changing buffer - this is flourocarbon not maxima
Why did I break from the 60/20/20 rule to extend taper and tippet? Because 30-20-10-10-20 is a mnemonic device I have memorized completely. In fact I've tied so many of these in several seasons of guiding that I don't even need to measure to come within an acceptable margin of error anymore. In fact I always extend fresh end tippet because I use palomar knots, the strongest knot, which take a little more line to create - 24" but you see the value of the mnemonic I hope.
Note: You can throw the 15# and either the 20 or 25# in the chest pack and not worry about tippets for a day of fishing even if gatormouth comes to the party.
For a long spey leader, I use the following:
30 - 60"
25 - 40"
20 - 20
15 - 20
12 - 20
10 - 20
On rivers like the Thompson:
35 - 60"
30 - 40"
25 - 40"
20 - 20"
15 - 20"
15' leader - (push the bigger diameter to the tippet)
Once again as long as you have the desired tippet spool in your vest you are good for the whole day.
Trout leaders (stillwater 9'):
25 - 40"
20 - 20"
12 - 8"
10 - 8"
8 - 8"
6 - 10"
4 - 14"
You can play with the line ratings and slide the lengths around but AJ was right - the 60/20/20 rule is about as complicated as one really needs to get for normal everyday fishing!
When I get some time I will put together a calc program, although any calc program will lack the merit of human logic and intervention which as you can see here is the most important element of the process.
I knew a guy who used a $20,000 CAD system to design a deck for his kidney shaped pool. He provided blueprints to the carpenter and told him to cut the wood according to his plots. The carpenter didn't like the idea, wishing to cut the wood as the deck went down to iterate the fit from one piece to the other. My co-worker argued thinking he was going to save labor costs. So the carpenter cut all the wood according to the fancy plan which was +/- .01 in the $20k computer. Well reality struck as the project got underway - half of them didn't fit due to compounded tolerances and angular imperfections in the non-digital domain so the project ended up costing much more.
My point - use the simple guideline, don't be afraid to use common sense to reach the real goal - a workable leader logic that you can remember and benefit from.
The greatest benefit of tying your own is the ability to refurbish a damaged leader while standing knee deep in the water. With the exception of extra-fine trout leaders, I never buy tapered one-pc leaders. Hmmm that probably explains why I never buy tapered one-pc leaders