Tim Ahrens scaling

.py

from dataclasses import dataclass

@dataclass
class Factor:
	x: int
	y: int

def interpolatedPosition(foregroundPosition, backgroundPosition, foregroundFactor,  backgroundFactor):
		interpolatedX = foregroundPosition.x * foregroundFactor.x + backgroundPosition.x * backgroundFactor.x
		interpolatedY = foregroundPosition.y * foregroundFactor.y + backgroundPosition.y * backgroundFactor.y
		interpolatedPosition = NSPoint(interpolatedX, interpolatedY)
		return interpolatedPosition

def interpolatePaths(thisLayer, foregroundFactorX, foregroundFactorY):
	foregroundFactor = Factor(foregroundFactorX, foregroundFactorY)
	backgroundFactor = Factor(1- foregroundFactorX, 1-foregroundFactorY)
	
	for path_index, path in enumerate(thisLayer.paths):
		for node_index, node in enumerate(path.nodes):
			foregroundPosition = node.position
			bPath = thisLayer.background.paths[path_index]
			if bPath:
				backgroundPosition = bPath.nodes[node_index].position
				node.setPosition_(interpolatedPosition(
					foregroundPosition,
					backgroundPosition,
					foregroundFactor,
					backgroundFactor,
					))


foregroundStem = 65
backgroundStem = 100
targetScale = 0.6

targetFactor = (foregroundStem - backgroundStem*targetScale)/(foregroundStem*targetScale-backgroundStem*targetScale)


layers = Glyphs.font.selectedLayers

layer = layers[0]
RSB = layer.RSB
interpolatePaths(layer,targetFactor,1)
layer.applyTransform([targetScale,0,0,1,1,1])
layer.RSB = RSB
layer.syncMetrics()