Interestingly enough, the FAA defines IFR and IMC but not specifically IFR conditions, which is what they use in the regulation I quoted (91.167). IFR conditions must be deduced from the combination of the definitions.
FAA definitions:

Instrument flight rules (IFR) means rules governing the procedures for conducting flight under instrument meteorological conditions (IMC) instrument flight.

Instrument meteorological conditions (IMC) means weather conditions below the minimums prescribed for flight under Visual Flight Rules (VFR).

At any rate, the intent of the regulation is that we plan to have at least 45 minutes of fuel in the tanks when we land at the Alternate. When do we plan this? Sure during preflight, but also during the flight, usually while flying the hold. We must leave the hold when the fuel equals the amount to fly from the Hold to the Destination and then on to the farthest filed Alternate. Then be able to fly 45 minutes after that at normal cruising speed.

 

Yeah that's part of the problem with this reg... if you wanted to split hairs. Better to just stay conservative.

 

Maybe it was an assumption, but I always figured if an alternate was required (123 rule), then the reg applies. Same applies to a second alternate (enough gas to reach the furthest alternate, not both). How could the reg possibly legally require an alternate, but not require having enough gas to reach it? But I get that the FAA is driven by strict, bureaucratic interpretation of the law that'd defy any common sense bias.

 

Looking back at the OP's question, quick and easy inflight planning, using an FMS for a tactical problem is something that could be better taught, depending on where and who has provided your training and guidance for your current jet. To the point of most posters responding to the OP, the FAR's provide a pretty clear plan for dispatch, re: fuel required, but leave a lot to the interpatation of how en route to decide how much is acceptable for a divert. The FMS that most of us use, I believe, has several functions which will compute something but none can tell you how much fuel you should have when you finally touch terra firma. The old adage, "Sh!t in, Sh!t out" regarding computations, absolutely applies here. Aviation is full of variables that the Administrator, weather office and company flight standards can't all provide guidance for. Sometimes you just have to point the jet in the right direction and use your gut as to how long you can loiter before landing somewhere you, your passangers, or your employer would rather you didn't. Thats why they pay us the big bucks.

 

I always took it to mean to determine the burn present position to your alternate + landing with :45 in the tanks = time to leave for your alternate. if you burn in to the :45 minutes declare min fuel and if you get to less than :30 that's emergency fuel.

 

You can burn into your alternate and reserve if you choose to do so legally. That being said you must be assured you have high confidence that you will land at destination as once you do that you are no longer able to make the alternate

 

For the most part, things happen in the holding pattern on the STAR. Normally an approach isn’t started until you have weather mins with a high confidence of getting in. It can become where, the destination is a better choice than any alternate.

If back in the stack on the arrival, number 20 or so, maybe it’s time for ‘uncle’ & bug out. A missed approach at 200’ then divert is relatively uncommon, normally the decision is made before that.