I am very new to Python but need to write a script to add 20% to a field and then round that number with the 20% to the next 50. I have figured out the calculations but for the life of me can't figure out how to put them together to get it into one function.

I've tried this in the code block:

def boom_length(boom): twenty_percent = ((!LENGTH_FT!) * 0.2) + !LENGTH_FT!) math.ceil(twenty_percent + (50 - twenty_percent) % 50) return twenty_percent

then in the expression:

boom_length(!BOOM_LEN!)

Thanks in advance for the help!

...and to explain the reason for these changes: