As in so many things, It's complicated.
I run different temp depending on the mold material, the style of bullet, the VOLUME of the bullet, whether it's got a HP or not, the alloy, etc.
I mostly use 1:40 alloy and as you might know, tin starts rapidly losing it's fill out effect at above 750 F, AND the oxidization really kicks in above this temp too.
And the hotter you go the more shrinkage you get so the bullets turn out smaller (which you may or may not want... for ex: straight sided grooves bullet molds extract better if you cast hotter, but not as necessary on angled side grooves) but a hotter pour tends to fill out better....
SO, using my bottom pour pot, I GENERALLY do my melt up to temp, then flux for impurities and to mix the tin then cap the melt with sawdust to prevent oxidization and run about 800F with an aluminum mold.
Casting fast to keep the mold and HP pins hot (preheating like I've described to you before and if the mold gets a little cold casting, preheating again with the dip method)
And I ALWAYS pour directly into the sprue hole as I've found I get too much weight variation (turbulent bubbles in the molten lead that solidifies?) and poor fill-out on HP mouth edges using the 'swirl' method of hitting the sprue plate cone first and let the lead redirect into the hole some use. Test what works for you.
Here's a HINT though, continuing the pour until you get a BIG puddle on top of the sprue plate for each cavity (but keeping it from overflowing into the next cavity) goes a long way to getting good base fill out as the flow pressure presses air out of the mold AND gives the core shrinkage a feed reservoir so not pinholes in the base.
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I will tell you a trick that if done carefully will help base fill out but if done enthusiastically/too much will ruin the mold. (For confident/savvy experienced tool using metal workers)
Take an extra fine file and hold it lengthwise at a 45 degree angle along the top edge of a mold block where it crosses the cavitie(s) and do ONE pass to break the edge and cause a TINY 'chamfer' (look up the word for those not familiar with it, look at a picture) of the corner. I only do one block, not both... this will act to help vent the base and give better fill out BUT if you make it too big you will get a feather of lead at high temperatures. What you are doing is making a vent like the vent line/scratches along the face of the mold. I have even done it with a hard backer with a piece of Wet-R-Dry sandpaper wrapped around it and a SINGLE pass the same as I described with the file just to break the edge of the corner, too.
Or, you might notice some manufacturers vent the top of their blocks and some do not, you can scribe new vent lines (intersecting the cavity) on top of a block that's not filling out that well on the base with a very sharp awl or scribe point and straight edge then remove the high ridge created from the displaced metal on both sides of the scratch too so the sprue plate fits flush again. I use a sheet of glass and lay a sheet of WetRdry on it (wet of course!) and put the block face on it and using two fingers gently slide it across the paper so it does not change the face of the block but DOES take off the high ridges created. DON"T tip the block or apply pressure because if you change the angle of the face you ruined it!
Also note that you scribe from the cavity AWAY because if you scribe to the cavity you will leave a little protrusion inside the cavity edge and it WILL show and cause problems. and if you were scribing TOWARD the cavity and slipped inside you would probably scratch or dig into the surface inside the cavity and that shows on every bullet too (Percussion University, the School of Hard Knocks is not the best way to learn, haha)
Again, this is a skilled technique, not for amateurs.
