@Pretty Mouth has it spot-on that you really need to get the manufacturer's data for breakers (MCB, RCBO or MCCB) as the usual current-time plots do not really give you a good idea of what happens once you enter the "instant" magnetic trip region. Some give you useful curves (like the Hager example) and others just give you a maximum value to base calculations on.
There are some generic limits that can be used as well, for example the On-Site Guide in Table B7 (page 146 of current 18th AM2 version), and usually any of the "standard circuits" in Table 7.1(ii) of the OSG are fine for domestic cases where you very rarely see above a couple of kA fault currents.
As
@Lucien Nunes says it can sometimes be necessary to look at how you limit the worst-case fault current for various reasons, not just cable adiabatic but also the OCPD breaking limit, etc. However, trying to increase R1+R2 is really your last resort if you find a problem, a better option where practical is to look at OCPD that can safely break the worst-case fault and/or is able to limit the I2t let-through energy to a lower value as well.
Fuses can be your friend here! Many BS88 fuses can break over 50kA PFC (compared to 6-10kA for MCB and often 18-25kA for many MCCB, though some do go higher to 70kA and above) and they do very well at limiting I2t often to orders of magnitude lower than a similar capacity breaker.
They are one-time only devices of course, and need skilled staff for safe replacement, but otherwise are cheap, reliable and effective if those two limitations are acceptable.