Hey Alex,
This might be a tad late, but you can reuse variables by defining them at the end of the query:
[//quantity:${ingestedMass}//] * 0.5 ^ ([//quantity:${elapsedTime}//] / (p * b / (p + b))),
where p = [//quantity:${phyHI}//], b = [//quantity:${bioHI}//]
Edit:
Alternatively, you can use the ReplaceAll[expr, rules] function:
[//quantity:${ingestedMass}//] * 0.5 ^ ([//quantity:${elapsedTime}//] /
ReplaceAll[p * b / (p + b)), {p->[//quantity:${phyHI}//], b->[//quantity:${bioHI}//]}]
~a different Alex
