Homebrew Macro help

I'm trying to get a macro for my home brew working. What I'm trying to do is have a modified roll [[1d20+@{modifiers}]] show the result and then calculate for me something based on the result. Specifically [[(1d20@{modifiers}-10)/2) which for this special attack tells me how big a certain bonus is.  The d20 results have to match or it's of no use to me. Do I need an API for this? If so, is there something already created that I can use? I'm a long time GM larning my way around Roll20. Have to say I'm loving it.

Recent emergent behaviour might be your salvation here, although you have to do it in reverse (ie show the original roll last): &{template:default} {{name=Roll}} {{Calculation=[[ ([[1d20]] + @{modifiers}-10)/2 ]]}} {{Original= d20 = $[[0]] }}

Thanks for the response. IT didn't work cut and pasted (cannot read property 'substring' of undefined) but when I typed it out it posted the same calculation for both. I got this working a bit better: &{template:default} {{name=roll}} {{Bonus=[[ floor([[1d20]]+@{Player 2|Longbow EXP}-10/2)]] }} {{Original= d20 = $[[0]]  }} It's calculated my bonus and  posting the original die roll. I couldn't get the "original" to include the modifier though, until this: &{template:default} {{name=roll}} {{Bonus=[[ floor([[1d20+@{Player 2|Longbow EXP}]]-10)/2]] }} {{Hit Roll= d20 = $[[0]]+@{Player 2|Longbow EXP}   }} which is doing everything I needed. Thank you so much!

I managed to get a query working inside it: /emas "Robin Hood"  Shoots Longbow  &{template:default} {{name=Shot}} {{Bonus=[[ floor(([[1d20-?{total penalties|0}+@{modifier} ]]-20)/3+3)]] }} {{Hit Roll = $[[0]]  }} {{Damage = [[1d8]] }}