The procedure in this case is that you apply for a 46 day visa. It is then up to the consulate whether they (a) reject your application, (b) issue a 46 day visa, or (c) issue multiple-entry 90-days-presence-out-of-X-days-validity visa. It is not under your control whether (b) or (c) happens. It is not uncommon that frequent travelers who have a history of sticking to the rules are issued visa with a long validity without asking for them in order to allow them to be more flexible.
However, the consulate should normally not issue visa that covers less than the period that you apply for. So you could apply for the full 90 days. Doing so does increases the probability of visa refusal, however - especially if you can't provide them with a good reason for what you need the 90 days for.
So the rule of thumb is: Just apply for what you need and be happy about it if you get more validity than you need. However, don't apply for more than you need, as it increases the risk of a visa refusal.
Note that it arrival and departure are 45 days apart, you actually need a 46 day visa as partial days spent in the Schengen area count fully.