Calculated Field for Activity Over a Month for KPI Percentage

Hi, I’m trying to calculate the percent of leads that were active over the past month as a KPI. I had to use a date field and narrow down activity by the month using this:

“periodToDateCountOverTime(count({s_t_sf_id}),{s_t_sf_most_recent_activity},MONTH)”

And then I took the initial calculated field and put it into this calculated field to try to get the percent of leads that were active. But, i’m unable to add it as a KPI. I feel getting the missing field well error. Is there a better way to re-write these equations or a fix to have it be KPI able? I tried adding pre-agg but it was erroring out…