While the portfolio doesn't offer short/intermeidate investment grade selection; you could choose the 5 year Treasury option.
I ran a $1,000,000 portfolio 30% stocks 70% 5 year Treasuries=success rate 86.4%. The program calculated for 30 years that:"A spending level of $34,888 provided a success rate of 100.0%". The program calculated for 40 years that:"A spending level of $29,302 provided a success rate of 100.0%".

I'm not an expert at numbers, but does this mean that a 70% fixed portfolio has a 100% success rate at 40 years if you use a 3% withdrawal rate, and same for 30 years at a 3.5% withdrawal rate? Thanks.