Mashing Question

I have heard that I should heat my mash water about 15 degrees warmer than the temp at which I intend to mash. however what if I have preheated my cooler up before i mash....say fill the cooler up with near boiling water and let stand for 15 20 minutes.....it doesnt seem that I would need to heat my water up as hot since the cooler would not be absorbing much heat. Any rule of thumb on how I should adjust water temp for mashing in such a circumstance? Maybe 10 degrees? or maybe less? I hope someone out there can give me some advice on this one.

I believe I have read on here something like 8 deg. Just give it a shot. If you are off 2 or 3 deg. one way or the other it wont kill you. If you are cold add some hot water if you are to hot add some cold.

I use this calculator for my mashes and have hit my temps pretty much exactly each time.

The exact calculation depends on grain temp, grain weight and water/grain ratio. My experience centers around 1.25 to 1.5 qts/lb with grain temps in the 60-70F range. Strike temp is usually somewhere around 165F with a preheated tun. So, it's like 15F higher. If the tun isn't preheated, it would be more like 178F. Download beersmith or beertoolspro for a free trial.

Not to hijack the thread, mostly on topic, but I just bought a stainless steel 10g mash tun kettle with a false bottom. Can I just preheat it on the stove with some water in it? What about heating my strike water directly in the mash tun and then adding the grain? Does that then run the risk of scorching?