That's a really good question. I don't know that shared custody is even defined anywhere. The guidelines just mention equal or nearly equal blocks of time. If 40/60 is nearly equal, then I suppose you could be treated as having equal time. But, the guidelines have an adjustment for up to 45/55 of 15% and 40/60 of 10%, so I'm guessing they would simply apply something like that.
As far as entering it in the spreadsheet, you'd probably just consider either the equal parenting time formula or the direct expense sharing equation and go from there. Parenting time isn't factored into the spreadsheet directly, so if a judge considers your case shared residency, they would use the shared residency equations.