Those are good guidelines.
I too, was looking to enter around the same time you mentioned, and exit well before heavy gamma kicks in.
As far as underlying movement, we can look at the history of relative price move% to get some idea of how much tendency the underlying has moved in the past, during this specific time frame, to see if there are any repeating patterns,
As far as the loss of value, on the short expiration, I think people tend to look in the wrong place for this.
There is a big difference between the rate at which an option deteriorates, and the amount of premium an option loses.
Most option sellers like to focus on the last 30 DTE, because time decay is the greatest.
But, the gamma risk, associated with that is the highest.
The thing that many people are missing, is that while an option may deteriorate at the fastest rate, during the last 30 DTE, it is not losing the most amount of it's value during that time.
There is a period between 75 DTE- 50 DTE, ( it is hard to pinpoint the exact spot, but generally), where a 10 delta option will lose 50% of it's value, and not have much gamma risk associated with it that far away.
An option is "born" with x amount of premium. By the the time it reaches T-30, it has already lost 90% of that premium.
So, it is losing it's last 10% at the fastest rate of decline, with the most amount of gamma.
So, this is sort of the worst spot to focus on for an option seller.
Also, with regard to these 2 different calendar strategies, while a "normal" calendar, is a "long IV" position, one can make a case that a "pre earnings calendar" can be a short IV position.
Look at the case where the long and short IV starts out at 45 and 43, and winds up at 95 and 50 IV....This does happen in some cases, and while the value of 1 vega is much greater, in the longer dated expiration, sometimes the front expiry IV gain is even greater than that.
But, during non earnings times, a calendar is a long IV position.
What is good about this trade is that the leg that you are long, is the first expiration post earnings.
This is the expiry where, eventually most of the money will flow into, and will reach the highest IV of all expirations.
This may not happen during the time frame of this trade, but the long leg will have a degree of "IV support".