Battery Reserve Capacity - how do you determine lower load capacity?
No.  First there are many many types of batteries, so arbitrarily defining Reserve Capacity, whatever that is really supposed to mean, as something related to a fixed amount of current like 25 A is absurd.  A CR2032 coin cell, for example, can't ever provide remotely close to 25 A under any circumstances for any amount of time.

Your "reserve capacity" is in units of current times time (charge), which is also the units used for normal battery capacity.  Normal battery capacity is usually expressed in A-h or mA-h, but this can be directly converted back and forth to your 25A-minutes.

You can as a rough first approximation estimate battery life time by dividing its capacity by the current you will draw.  However, batteries are complicated, and this is far from a ideal relationship.  If a battery is rated for 10 Ah, for example, then you will probably get close to 2 hours of use at 5 A and 10 hours at 1 A.  But, currents both significantly smaller or larger can affect the overall capacity significantly.  Large currents usually degrade capacity.  At very small curents it can also be less because the self-discharge becomes significant.  Temperature can also have a major influence on overall capacity.

The best way to estimate length of time a battery will be good for at a particular
current is to examine the datasheet carefully.  Look at the graphs of capacity versus current, what the cutoff voltage is used to determine "dead" and how that relates to your application, and the temperature this was all assumed at.  The age of the battery will also matter, and the number of charge/discharge cycles if it's a rechargable type.  Then realize these graphs generally show typical, not minimum guaranteed, values.  Derate somewhat for worst case.

For particularly tricky cases, contact the manufacturer for guidance, and/or do a bunch of your own tests.