Boil away, no worries, DMT will not be destroyed.
As minxx said, there is one alkaloid in mimosa, yuremamine, which is destroyed by the addition of any acid or heat. This alkaloid is what makes cold-water-infusions from mimosa work orally without extra MAOIs, but the dosage needed for this to work are huge (like lets say 25g+, as opposed to 3-5g needed, when mimosa is boiled instead and later MAOI is added). So dont worry about dmt being destroyed, just keep brewing, all is fine
BTW, regarding amount of water per wash, I always go by eyesight, cover the mimosa with water and a bit more and do it 3x (or 3x + a long soak) but anyways if you want people to give you feedback, you might as well say how many grams mimosa you are using, because just stating that you are doing 1 liter isnt enough info (1 liter per 10 grams or 10000 grams mimosa?)