If you simply want to know how they figured out the maximum property weight for each runic tool ignore the rest of this post (I have no clue).

If you want to know how much weight reforging assigns to a specific property intensity, there's no need.
[General] - Reforging Question... Runic Weights? | Stratics: A Career Path in Gaming
[Imbuing] - Reforging. How to | Stratics: A Career Path in Gaming
Semmerset gave us all the info we need in those threads. Imbuing allows us to custom tailor exact property intensities/number of properties. We're given the two variables ( base property weight total item weight ) we need to figure out exact property intensities.
Reforging is a more focused and updated version of the runic crafting we've always had. Since the intensities/properties are still random we don't need to know individual weight per percent intensity. We only need to know the maximum intensity per property. Since the property intensities are still random we set the budget (choose the right runic tool) high enough to allow the rng to roll a maximum intensity property.
This shows each hammer's single property budget, total budget per hammer and how many properties each can produce. Powerful,structural,fundamental add 20/40/60 to a hammer's budget.
To increase the chance of a single max property you'd use the lowest end hammer with the budget you need.
For -15 mage weapon:
Find the lowest end hammer that can produce the intensity you want.
Copper Runic with powerful reforge in this case.
140 (copper's max single property intensity) + 20 (powerful reforge) = 160.
Any hammer with a single property budget of 160 or more can produce a -15 mage weapon. There is chance the copper runic will add two additional properties. Each property adds 40 points to the total budget of the hammer and the points will be split between all three properties.
If we wanted to try for a -15 mage weapon with 40 ssi we need a hammer with atleast a two property 320 point budget. An agapite hammer + powerful reforge would have a chance to make it, but it can also roll three more properties.
-15 mage wep takes 160 points
40 ssi: same as the mage wep (copper hammer and powerful reforge) 160 points