Save the kern value from the last pair you were on, for quick application to your current pair (with the Page-Down key). Install "store..." as a start-up script, and "set...py" as a normal script in the script folder.

set_latest_kern_value.py

# menuTitle: Set the Latest Kern Value
# shortCut: pagedown

from mojo.UI import getDefault
import metricsMachine as mm

KERN_VALUE_LIB_KEY = 'com.ryanbugden.storeLatestKernValue.kernValue'

def get_kern_group(f, glyph, side):
    if side == 'left':
        groups = f.groups.side1KerningGroups
    if side == 'right':
        groups = f.groups.side2KerningGroups
    for key, value in groups.items():
        if glyph in value:
            glyph = key
            break
    return glyph

def convert_to_group_pair(f, pair):
    l = get_kern_group(f, pair[0], 'left') 
    r = get_kern_group(f, pair[1], 'right') 

    return (l, r)

f = CurrentFont()
cp = mm.GetCurrentPair()
group_pair = convert_to_group_pair(f, cp)


try:
    value = f.lib[KERN_VALUE_LIB_KEY]
    f.kerning.update({group_pair: int(value)})
except:
    self.f.lib[KERN_VALUE_LIB_KEY] = 0


# if group_pair in f.kerning.keys():
#     f.kerning[group_pair] = int(value)
# else:
#     f.kerning.update({group_pair: int(value)})
    

store_latest_kern_value.py

# menuTitle: (Start-up) Store Latest Kern Value

from mojo.UI import getDefault, setDefault, preferencesChanged
from mojo.events import addObserver

'''
Ryan Bugden
23.03.23
'''

KERN_VALUE_LIB_KEY = 'com.ryanbugden.storeLatestKernValue.kernValue'

def get_kern_group(f, glyph, side):
    if side == 'left':
        groups = f.groups.side1KerningGroups
    if side == 'right':
        groups = f.groups.side2KerningGroups
    for key, value in groups.items():
        if glyph in value:
            glyph = key
            break
    return glyph

def convert_to_group_pair(f, pair):
    l = get_kern_group(f, pair[0], 'left') 
    r = get_kern_group(f, pair[1], 'right') 

    return (l, r)

def get_kern_value(f, pair):
    kern_value = 0
    if pair in f.kerning.keys():
        if f.kerning[pair] != None:
            kern_value = f.kerning[pair]

    return int(kern_value)

    
debug = False

class storeLastKernValue:
    
    
    def __init__(self):

        self.prev_kern = ((), 0)
        addObserver(self, "pairChanged", "MetricsMachine.currentPairChanged") 
        
        
    def pairChanged(self, sender):
        
        try:
            from mm4.mmScripting import MetricsMachineScriptingError
            import metricsMachine as mm

            self.f = CurrentFont()
            cp = mm.GetCurrentPair()
            group_pair = convert_to_group_pair(self.f, cp)

            # if the pair changed, store the previous pair value, and set it as the new previous
            if group_pair != self.prev_kern[0]:

                if debug == True:
                    print("Previous kern pair and value:", self.prev_kern)

                # register the previous kern value
                self.kern_value = get_kern_value(self.f, self.prev_kern[0])
                self.f.lib[KERN_VALUE_LIB_KEY] = self.kern_value

                # if the current pair isn't none, set it to the previous now
                value = get_kern_value(self.f, group_pair)
                self.prev_kern = (group_pair, value)

        except (MetricsMachineScriptingError, ModuleNotFoundError) as se:
            return

        if debug == True:
            print(f"Registered {KERN_VALUE_LIB_KEY} default right now:", self.f.lib[KERN_VALUE_LIB_KEY])

        
    
storeLastKernValue()