Hmm, once you've linked the records together, you can use lookup fields to pull the values over and use formula fields to help you get the percentages
To do this, you would:
1. Create a linked field to the same table
2. Create lookup fields to pull the data you need over, e.g. "Reach"
3. Create a formula field to display the percentage change. When you create a lookup field, the field name will be something like "Reach (From Table 1)" or some such, so your formula would be something like:
IF(
{Reach (from Table 1)},
(Reach - {Reach (from Table 1)}) / Reach
)
I've something up here that does this for you
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Getting the records to link automatically is slightly trickier but still doable. Given that this is a once a month thing it might not be worth the effort though?