Scaling

Hi everyone...
I am bit struck so thought to get a little help from u guys...

I am bit confused which function shall i use to scale something from 0-100%...

Tph FEO COil Actual FEO
1000 10% 10%
500 5% 10%

This is short example to make life bit easier as u all can see above FEO COil % decreased to 5 % whereas Actual FEO is 10%
It Shouldnt goto 5% and stay at 10%

If someone could help me out with this please what function shall i use to make some logic...

regards

Adi

Actual = Tph / 100
If actual < 10 then actual=10

But lets suppose if TPh goes to '0'
it will go to infinity and crash the PLC..

0/100 = 0, not infinity.