on the reel I am going to get it says for 15 pound braid you need 145 yards I found 20 pound braid 100 yards is that too little?  Or do I not fill the spool?

Always fill the spool. 
Use about 20-30% of the spools capacity with cheap mono as backing. 
 

 

 

 

Mike

Fill the spool to 1/8" of the lip.  Use backing to make up any difference.

Comparing spool capacity, line diameter is more important than line test.  

Here's a really good spool capacity calculator.  

 

The Advanced capacity calculator allows estimating capacity when stacking lines of two different diameters.  

Just now, bulldog1935 said:

Comparing spool capacity, line diameter is more important than line test.  

Here's a really good spool capacity calculator.  

 

The Advanced capacity calculator allows estimating capacity when stacking lines of two different diameters.  

Darn it all - I was about to point him to the advanced page myself...was typing it up when you sneaked in.

?

I can't turn around without using it these days.  As much as I hated differential equations, most recently used it to tie two different leftover quality braid pieces (about 50 yds) for backing this finesse braid.  

10 minutes ago, bulldog1935 said:

?

I can't turn around without using it these days.  As much as I hated differential equations, most recently used it to tie two different leftover quality braid pieces (about 50 yds) for backing this finesse braid.  

Ya, I've become virtually dependent on it myself for both spinners and BCs since I mono-back everything - even when I'm using mono or hybrid as mainline.

3 hours ago, MN Fisher said:

Ya, I've become virtually dependent on it myself for both spinners and BCs since I mono-back everything - even when I'm using mono or hybrid as mainline.

Nope, while I used to be a math geek, I'm not doing anything like that to fill a reel. If you have 2 of the same reel (or 2 with the same size spool), just fill one with the working line first, then the backing, and then fill the one with the other, OR get one of those little line counters. We use them to set trolling lines.