Searcher,
Not sure if you are in ER, or your comment meant that you could get to ER with inflation plus 2%, but if the former, then I would have some issues with your plan.
For a definition, my feeling is that a conservative investment generally means one that doesn't have a lot of upside or downside. Thus, you can pretty much count on the return you are going to get.
In that case, you need to 'lock in' something that will either be explicitly inflation adjusted (TIPS) or else you need to assume an inflation rate (3% is the historical average) and subtract it from your 'locked in' return to get your real return.
Then, if you are in ER, you will need to subtract the amount you intend to 'bleed' off your portfolio each year for living expenses. Most people are somewhere between 2 and 5%, (remember that a 1% SWR means you're stashing a million for every 10k a year you take -- not exactly in the province of most people.)
Then I feel pretty strongly that you'd want to have 1% left over if you can for some forward momentum or growth over time in your portfolio. Over 50 years, a lot can happen, and if you have built in an assumed bit of positive growth, you may be able to cover something unexpected.
Finally, don't forget your investment management fees -- something between .2% for traditional index funds at Vanguard and 1% for the lower-cost actively managed funds. Maybe .5% covers average cases with room for fund trading expenses. (These expenses only apply to funds or things you might buy and sell without waiting to maturity (bonds you sell early, or stocks).
Long way of saying I think you need at least 8% from your 'super safe' investments: 3% for inflation, 4% to withdraw for expenses and 1% to grow on.
Anything less and you'll be eating into principal/ not keeping up with inflation. Subtract from this 8% any amount less than 4% on your SWR, ie. if you only take a 3% swr, you can lock down a 7% interest rate and be ok long term.
Risky investments need to have an expected value of at least 8%, for the same reasons, but because they are risky, you can come out ahead or behind at the end of the day. But locking down an overall portfolio return lower than your SWR plus Inflation guarantees you'll be trashing your portfolio, so I don't see how that can be a part of a plan. (the 1% extra is gravy -- you may not feel the need for it.)
