I really don’t think it’s all that hard to determine when to begin your fast - yes, it can vary by a matter of minutes, but obviously receiving the Eucharist isn’t going to happen right at the start of Mass. It seems reasonable to say that, if you begin fasting 20-30 minutes before Mass, you are within the required limits. This doesn’t mean you can’t do more. I also don’t think that there’s any reason to be critical of those who choose not to practice more than the minimum. Different things have meaning for different people. It seems to me (though I admit I am no expert) that the fast is simply a matter of respect for what you are about to do. It’s not a matter of keeping the Body and Blood away from other consumed foods - otherwise, we’d be fasting after Mass, too. If a person follows the requirements of the Church and receives Communion with respect for the sacrament, it seems to me that that person is fulfilling everything that God would wish them to do. If there’s more to it than that, please tell me - I always want to learn more!