The simplest answer is that if your current system appears to be sized correctly, merely replace like with like.
If the existing system is 12-15 years old, the higher performance efficiencies of current HVAC systems would allow you to downsize capacities 10% and still be of sufficient capacity.
Unless you have reason to believe (actual evidence, not supposition) that your air distribution system (ductwork, etc.) is grossly mis-sized or in poor condition, you should have no worries over that issue. Even if the existing ductwork is wildly undersized, a knowledgeable engineer or system designer can make the proper system adjustments to adequately utilize the existing ductwork.
There are no "rules of thumb" regarding residential HVAC. There are too many variables to permit you to say that 400 sq. ft. per ton is acceptable, or that 600-750 sq. ft. per ton is correct. Either of those ranges, or neither, may be correct. Climate, fenestration, envelope insulation, infiltration, occupant load and usage, lighting and appliances, etc. all are factors that must be included in any load calculation. Unless you are doing cookie cutter designs, each project is a prototype.
And while a ton is 12,000 btuh, there is also a latent cooling requirement that varies considerably between applications that must be taken into account. [For those interested, the sensible portion of most equipment cooling "tons" is generally anywhere from 70-80%, the remainder is latent cooling. Some systems will vary more than this which is why the equipment selection must be tailored to the particular application]. For proper occupant comfort, as well as protection of wood furniture, artwork, paneling, and the prevention of mold and mildew, etc., indoor humidity levels must be controlled.
Manual "J" is a simplified load calculation checklist/spreadsheet that was designed to allow HVAC Contractors to make better system assessments than just "eye-balling" the application. It is essentially a "fill in the blank" project that takes less than an hour to complete. If properly performed, it will be suitable in many, though not all, situations. It is not the load calculation that an engineer would utilize, nor is it especially helpful for retrofits where other factors make also come into play.
Are there Utility Company rebates that may be available and may factor into your decision? Do you have a poor installation of your condensing units, or is the return air path restricted? Is the ductwork run in non-conditioned spaces and has inadequate insulation? Are the supply air registers adequately sized and properly located? Etc., etc., etc. Only a knowledgeable observer can give proper opinion over these and other issues. Manual "J" will not help you there.
There are some smart Contractors out there that can give you the correct answer, though they may be hard to find. A couple hours from a mechanical engineer making a site visit would be my recommendation, as they may also observe other issues that you were not previously aware of including electrical and plumbing, as well as code deficiencies. An engineer also would be a help in performing a final inspection prior to payoff.
Sometimes a little expense upfront for an expert opinion will pay off in the long run.
Hope this is helpful.
As always, offered only as opinion
