the easiest method is neither a re mash is the easy option.
If your doing a 90 min mash do the first 60 mins as normal and drain the wort off into bucket then add further water at 80 degrees stir and leave 15 mins, drain off add last water at 80 degrees and drain to your final volume.
I split my amounts of water into 14 litres, 6 litres and probably last one will be around 6 litres. It all depends on your brew length