You actually won't. A good graphing calculator will treat "ab/cd" as "(a*b)/(c*d)" but "a*b/c*d" as "((a*b)/c)*d" (or sometimes as "a*(b/c)*d") and actual usage by engineers and mathematicians aligns with the former not the later. You actually can't enter the expression in a non graphing calculator typically because it won't support implicit multiplication or variables. While you can write any formula using PEMDAS does that really matter when the majority of professionals don't?
Actual usage typically goes parentheses, then exponents, then implicit multiplication, then explicit multiplication and division, then addition and subtraction. PEI(MD)(AS) if you will.