My 401(k) doesn't have a "Total Market Index" - Do I build one instead?

My 401(k) allows me to choose some index funds as investments, but there is not a “Total US Stock Market” option - instead there are “Small-Cap Index Fund,” “Mid-Cap Index Fund,” and “Large-Cap Index Fund” options (there is an “International Index Fund” and a “Bond Index” as well, so those 2 of the 3 fund portfolio are clear).

I’d like to build my own “Three-Fund” portfolio because there is not a Target Date INDEX Fund and the expense ratio is around .4% for the Target Date Fund that is available (vs. the incredibly low expense ratios for the index funds).

Will I need to build a “Five-Fund” portfolio in this case?

  • ~10% Bond Index, 
  • ~36% International Index, and 
  • ~54% broken between the Small, Mid, and Large-cap Index Funds?

If so, should I just split that 54% equally between Small, Mid, and Large-cap Index Funds? Or should it be more like ~27% Large-Cap Index Fund, ~14% Mid-Cap Index Fund, and ~13% Small-Cap Index Fund (proportions that seem to be close to how the 2055 Target-Date Retirement Fund option in my 401(k) is balanced)?