The only parameter which cannot be directly measured is the ELP - effctive lens position
First order formulas estimated ELP using AL alone however the relationship between AL and ELP is not linear. Only 10% of large eyes have large ACD and 20% of small eyes have small ACD.
Second order formulas took both AL and K into consideration but did not perform sig better
For normal eyes SRK/T Holladay I Hoffer Q are OK For very long or short eyes 7 parameters need to be taken into consideration (AL, K, PreopRef, horizontal corneal diameter, age - dt lens changes) = Holladay 2