Treating investment gains over a time period of under 10 years as negligible is a perfectly prudent thing to do when planning a retirement which is going to span multiple decades. I do agree it is somewhat less prudent at a time scale of 20-30 years or beyond.
Your retirement goal is 25 * 12 * x.
If you save 50% then each month you spend x and put x in the bank. At that rate, it will take you 25 * 12 months (25 years) to get to your goal. Counting 1/3 of 1% returns each month, it would take you 208 months (17 years is 204 months). So returns would shave 7-8 years or ~30% off your working life.
If you save 75% then each month you spend x and put 3x in the bank. At that rate, it will take you 25 * 4 months (8 years is 96 months) to get to your goal. Counting 1/3 of 1% returns each month, it would take you 87 months (7 years is 84 months). So returns would shave off ~1 year or 13%.
Your retirement goal is 25 * 12 * x.
If you save 50% then each month you spend x and put x in the bank. At that rate, it will take you 25 * 12 months (25 years) to get to your goal. Counting 1/3 of 1% returns each month, it would take you 208 months (17 years is 204 months). So returns would shave 7-8 years or ~30% off your working life.
If you save 75% then each month you spend x and put 3x in the bank. At that rate, it will take you 25 * 4 months (8 years is 96 months) to get to your goal. Counting 1/3 of 1% returns each month, it would take you 87 months (7 years is 84 months). So returns would shave off ~1 year or 13%.