Just choose the same or higher wattage power supply at 12V. Since the original supply is 90W, I'd suggest buying one of the very common 12V 100W or 125W models. Use a higher-rated supply if you intend to run it in hot environment, e.g. without air conditioning, right after taking it out of a hot car, exposed to sunlight, etc.
That's about it, really.
If you want to dig a bit deeper:
Depending on circumstances, the efficiency of the internal buck power supplies in the laptop will vary with the input voltage. Given that the input voltage range is 12V to 19V, it may well be that the sweet spot for lowest heating in the laptop is around 15V. It's hard to know outright, depending on how the power supplies are implemented inside the laptop, but generally at lower input voltages, the conduction losses grow, whereas at higher input voltages, the switching losses grow. Whether the optimum is inside of the 12V to 19V range or outside of it depends on how good a job the designers did.
The typical way you'd measure it is by running some fixed benchmark continuously on the laptop, representative of your typical use case, with the screen on, and plot power consumption (voltage x current) as a function of input voltage. The input voltage with lowest power consumption is the one where the power supplies inside the laptop waste least amount of power due to inefficiency. On the other hand, the difference between the best and worst power consumption in such a test may be so small as to make it no concern. That's up to you.
In practice, you won't be wanting to need oddball power supplies, so the common values of 12V and 15V would be the only choices worth considering.
