Water needed for 5 gallon herms system

I am getting ready to brew my first 5 gallon batch with my Herms system. once I figure out the water to grain ratio how much heated water do I put in the mash tun to start?  My HLT will heat the Herms coil and my boil kettle will serve to heat all the mash/sparge water.  I just need to understand how much water to start off with in the mash tun so I can add the grains and then slowly fill the mash tun with water from my boil kettle?  Thanks.

When I had first started with my herms, I let Beersmith calculate my mash volume. I tweak the water/grain ratio to approximately 25% more sparge than mash volume. Also, select the equal batch option. This brings the ratio closer. It does for me. I have a two burner setup so I have the luxury of heating the mash and liquor tuns simultaneously.
I hope that helps.

Thanks for reply.  The amount beer smith says to put in does that all go into the mash tun at once while adding the grains?  I have a two burner system as well.  The herms coil is covered with heated water and i can use the pump from my boil kettle to pump to the mash tun.  Lets say i need 6 gallons total.  Do i heat 6 gallons in my boil kettle then transfer to the mash tun and then add grain?  or is some of the water keep in the boil kettle while the mash tun recirculates in the herms coil in the HLT?

The new version (2.3) has a Mashtun Addition field in the equipment profile. This is where you add all water volumes needed for your HERMS or RIMS coils, as well as foundation water under the false bottom, if needed. That way, your mash water ratio stays the same, regardless of batch size or grist weight. Mashtun Addition water is considered recoverable wort. Any wort that won't make it to the kettle should be in the Lauter loss field of the equipment profile.

Thanks for all your help.  I will give beersmith try.