im going insane

im reading harrington on online cash games and its got to the part of pot odds, im familiar with pot odds and he talks about the chance of making his hand on the river after the turn has been dealt when he has a flush draw (9 outs) he sais about how his going to win 0.20 of the time and lose 0.80 of the time which yeah is right but on the next page he sais about having pot odds higher then your hand percentage, the pot odds given are 2.1 and he then says "in previous example the odds of making our hand were 4.1
then puts
4.1=37/9
but i dont get it because this is wrong the odds of making his flush draw with one card to come are 5.1 being about 19% someone please tell me im not going mad or read it wrong ahhhhhhhhhhhhhhhhhhh

You're post is very tough to understand, very jumbled. I think you're trying to say that the book says he's 4 to 1 to make a flush with 1 card to come. 19% is a 4 to 1 shot. 81/19 = 4.26 to 1. the 19% is a very rough estimate, plus is very close to 4.1 to 1 he's correct. You think a flush draw is 5 to 1 because 19 goes into 100 5 times, but this is incorrect because you have to subtract the percentage from 100 then do the division.

ohhhhh god i didnt know that i learnt how to calculate my stuff through phil gordons little green book where he teaches the rule of 2 and 4 thats the method i use but didnt know that you have to subtract the percentage from 100 before you do the division. Thank you so much ive been sitting here going crazy thinking harringtons wrong but i knew it couldnt be lol.

The 2 and 4 is a good way to get a fair estimate on where you stand. And just so you know on the flop when you multiply by 4, the percentage you get is that you make your hand by the river, not the turn. So it would be best if to use the rule of 2 on each street, unless your facing an all in on the flop.

Also, always remember at that point there are only 46 cards left!