I think that its likely that there are many DL passengers traveling on the flight from FRA-BOM and they are probably used to having passengers making this short connection. I suspect they often hold it for a short time if their incoming flights are late. There are only a few incoming DL metal flights. I'd be tempted to go for it, but you could probably get DL to let you spend 24.75 hours in FRA, even though that's over the 24 hours limit for it not being a stopover. That way you'd be sure to make the connection.