Hi,
There would be too many factors to give an accurate answer I think.
How well your viv held heat, the ambient room temps etc woud all change the answer drastically. Also what the other end of the enclosure eneded up at when your hot spot was at the desired temp - it may end up too hot or cold.
I would say buy a good thermostat and a high wattage bulb, make sure it is well guarded to protect the animal and see what adjustments you need to make once that has settled down for 24 hours or so.
dr del