SharePoint Calculated field help

I have a sharepoint list that has a calculated field based on the values of other fields which is not operating exactly how I would like, I probably need to understand a couple questions about the field.

Here is the calculation
=IF([Pursuit Health Override]="green","green",IF([Pursuit Health Override]="yellow","yellow",IF([Pursuit Health Override]="red","red",IF(ttlrating3=4,"blue",IF(ttlrating3=3,"green",IF(ttlrating3=2,"yellow",IF(ttlrating3=1,"red",0)))))))

So basically the logic should be if Pursuit Health Override <> n/a then set this field to the value of Pursuit Health Override
 
Else
If ttlrating3 = 4 then set value to blue
Else
If ttlrating3 = 3 then set value to green
Else
If ttlrating3 = 2 then set value to yellow
Else
If ttlrating3 = 1 then set value to red

SO a couple things, I would like to change the first three IFs in the logic to one if possible saying in value is n/a set to Pursuit Health Override

The final part of the issue is that many times the value ends up as 0 which causes problems because I do some workflow work where I store this value into another text field called pursuit health previous and if the value is 0 the pursuit health value gets set to float;#0 which I don't know what is...