Cranking Pulse Widths:
The cranking pulse width at -40°F should be about 3 to 5 times the 170°F value.
If you over-estimate the correct values, you WILL flood the engine.
To begin, set the -40°F cranking pulse width to about 88% of your "upper" req_fuel value, and the 170°F cranking pulse width to about 23% of your req_fuel. Those should get you 'in the ball park' for starting.
Once you have the engine running, you can tune the cranking pulse widths with small changes, moving them up or down together, and check it over a few days worth of starts before deciding which direction to go next. Remember that the cranking pulse widths need to be with ~0.5 milliseconds of the optimal value at both -40ºF and 170º F.
You move them up and down together, but not by the same amount. You want to keep the cold cranking pulse width about 3 to 5 times the hot cranking pulse width.
For example, suppose your reg_fuel was 14. Then you might start with a cold cranking PW of 12.3 and a hot cranking pulse width of 4.0. Suppose you try this and see it is too rich, and your temperature when you tried it was 40°F. So you need to lean it out (after making sure the engine hadn't flooded, in which case you would have to clean the plugs). You might try reducing your cold cranking pulse width by 0.8 milliseconds, to 11.5. You don't want to also reduce you hot cranking pulse width by 0.8. You really want to reduce it by the proportional amount you reduced the cold pulse width, which would be 0.8*(4.0/12.3) = 0.26 = 0.3. So you would reduce it to 3.7.
This maintains a reasonable ratio between the pulse widths. You don't have to calculate it each time though, just adjust the hot number by about 1/3 of the cold number, and you should stay 'in the ballpark'. Conversely, if you had to richen the pulse widths, you might add 0.8, and 0.3, etc.
Note that even in a 'piggy-back' application, do not set MegaSquirt®'s cranking pulse widths to zero. This will cause unpredictable pulse widths up to 13 milliseconds. Instead, set them to 0.1 milliseconds in that application. This allows very little fuel to flow.
It is very easy to flood the engine, especially when cold, without realizing it, and this can confuse your start tuning quite a lot. So it's better to start with low pulse widths and work your way up in small steps (lean cranking is much easier to recover from),
As you get closer to optimal cranking pulse widths, you can adjust just one of the hot or cold pulse widths a bit to fix particular starting issues - for example, if you have hot start issues, warm the engine up fully (ideally by driving, not idling) then shut it off and adjust only the hot pulse width slightly to get the best starting. You can do the same with cold start, but you can only do it on 'true' cold starts, which means that: a) you have to wait for a cold day, and b) you only get one test per day (starting it and shutting it off right away still warms the combustion chamber, etc, and basing your tuning on that will throw you off).
If your engine doesn't respond while cranking at all, try disconnecting the serial cable to the laptop from your MegaSquirt® controller. The power supply from the laptop may interfere with your MegaSquirt® controller at times, making the engine impossible to start. If you try to datalog a cranking attempt, and the datalog is blank, or you are getting resets (secL keeps dropping back to zero before reaching 255), then you likely need to disconnect the laptop while cranking. You should be able to reconnect after the engine is started, and tune/datalog normally, without any issues.
Most engines are quite sensitive to the cranking pulse widths, so you may need both pulse widths to be within about 0.5 milliseconds to start well under all conditions.
Make sure you have power while cranking, otherwise you are starting on your prime pulse, and will go crazy trying to tune it,
Keep the prime pulse short, less than 4.0, and ideally around 2.0, while tuning the cranking pulse widths.
Make sure you are not in flood clear mode while cranking,
Datalog your starting attempts using TunerStudioMS, and study them carefully, you will learn a lot,
Don't use EGO feedback to guide you on cranking pulse widths, it won't help you at all, and may confuse you a lot. Instead, use your subjective sense of how long it takes the engine to start at any temperature (you are seeking to minimize this, obviously)