Can remember grandma saying was sweet spring water aka soft water, they had a spring on farm that was very soft water so no mineral deposits in kettle from boiling.
Use hardwood ashes only, soft wood carries over to much of the burnt resins from the pitch. Grandma would collect the ashes a few times a week and put into a barrel on back porch , she also had an old iron kettle on back of wood cook stove that she would occasionally collect small batches of lye water by half fill the iron kettle with hard wood ash and top to about an inch of water over the ashes and boiled it about 30 to 40 minutes. Let cool and settle then skim the lye water off the ashes and store in glass jugs she had. She would put the ashes in with rest of ashes in the lye barrel out back.
The main barrel on back porch had a hole drilled in side to hold a stopper , roughly 3 inches of gravel in bottom and 3 to 4 inches of straws packed on top of that then filled to about a hands width below top with ashes. Slowly fill and top up slowly with rain water of the soft spring water until ashes were fully soaked and just show water at top of ashes it would sit with with cover on barrel and stopper in side for a better part of a day. Next day grandpa would tip the barrel forward and slide a 1 inch board under bottom at back to tip it forward to drain, put pail under front and pull the stopper it slowly runs or trickles out so if you measure how much water you added you will know about how much will run to be collected. Do not add water while it is draining, once it is drain replace stopper and top up with more water and repeat. The water can be boiled down to collect a concentrated lye, when rendering it to a concentrate grandma used a potato to test if dropped and sank to bottom of the lye it wasn't ready if it floated it was strong concentrate and done and would need little dilution for use for soap if potato very slowly sank and seemed hoover down in the lye it was done ready for use or storing.
With a ph meter you will be able to tell if collecting a good lye it should be up in 12 to 13 PH and additional collection will drop in pH. Would see grandma test to see if she had lye by dipping finger in and rubbing together lye on fingers takes and eats away at the dead skin on surface almost instantly and give a slick almost slimy feel because of the extreme alkalinity ( i would suggest a ph tester instead of fingers ).
A few more bits of advice do not pour water into lye concentrate instead pour the lye concentrate slowly into water, do not try using aluminum or copper containers to collect and store lye concentrate. for easier storage get some shallow plastic trays , in a well ventilates area pour the concentrate in them and blow air across to evaporate the water out and collect dried crystal store in glass or plastic sealable jars. Lye is very caustic keep pets and children away from your work area. wear eyes protection and get some good rubber cleaning gloves that come up arms.
If you are in tropics you have a perfect source of a high potash coconut fronds and shells