In the above Rule of Thumb, the option that is correct is If existing software gets us 75-80% of what we need, we should probably buy.
Note that in the sales of good and services, when there is an assurance that a software has about 75-80% of what we need, we should probably buy because it may be hard or impossible to see more than than.
Conclusively, A 75-80% is close to a 100% and this also tells you that the software can also meet the requirement of what you want to do.
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