Pretty sure ya can get a free 21 day trial version of Beersmith at beersmith.com
Find out these variables if ya can, the weight (mass) of your mash tun, the volume of it, the wt of the grain... Beersmith will want to know all of this when you set it up, you have to set up your equipment and really for the few minutes it takes you can have piece of mind. So then as people suggested preheat your tun... I just put hot water from the tap in, a gallon or so, put the lid on and leave it there while I grind my grain and stuff... let it sit and heat up so the coolness of it doesn't soak up too much 'strike' water heat. Heat up your strike water and get it going, probably 2.5-3 gals, dump out the preheat water in your mash tun, put in your grain and pop a thermometer probe in the grain bed... I keep it covered during this always trying to keep it as warm as possible but it doesn't really matter, just get a stable grain bed temp, put it in beersmith. Then run the infusion tool, put in the variables discussed above and it will tell you your strike temp... My preheated mash tun had about 8.75# of grain yesterday and the temp was about 80 degrees... I put in about 3 gallons of water at 162 to hit 150 and I nailed it, put the top on my Igloo and 60 min later it was 149 in there..