It is atypical for a mash pH to come in that low using distilled water and that malt bill.
While it's not impossible for the mash pH to be that low using that grain bill it is improbable.
First, the pH of RO water is a) unimportant to brewing by-and-large and b) is often lower than what people typically quote (7) since CO2 from the atmosphere will dissolve into RO water.
Second, I don't believe the pH strips you are using are adequate to test the pH of your mash. the pH 4-8 strips are strips with which I am unfamiliar, but the 4.6 - to 6.2 strips are likely the precision labs pH 4662 strips that my LHBS sells.
The reasons I believe these strips to be inadequate for mash testing are
a) Your strips are of unknown accuracy - the EMD strips have shown that they have an error that causes them to read 0.3 pH units lower than the actual pH of the solution - the precision labs strips (which are much cheaper than EMD colorpHast strips, if we're using price as a quality determinate) may very well have an error of which you are unaware. Furthermore, the color index on the Precision Labs would indicate that the accuracy of these strips is +-4 which is not adequate for any brewing application, IMO.
b) The gradation of color on the precision labs is not adequate for one to draw any strong conclusions. A color gradation of yellow to brown is not a range in which definitive conclusions could be drawn. Viewing a strip under a tungsten bulb vs a florescent bulb could very well make the difference of .8 of a pH unit.
c) I've actually had the Precision Labs strips change colors in their sealed container, and not to a color that would indicate that there was excess moisture in their container, rather a color that would indicate the strips had dried out. The strips start as a very vibrant yellow and when I looked at the container after having purchased the strips 6 months prior the strips color pad (for lack of a better term) was a very pale almost white.
Having said all that, I think the best course of action for your water when brewing pale beer would be to cut it down with some RO - just like you're doing and add back some calcium in whatever way you see fit. When you don't have a pH meter it is difficult to get your mash pH in the right range because you don't know when you're there. My only recommendation is to NOT add additional alkalinity (like chalk, baking soda or calcium hydroxide) to your water until you have an accurate means of measuring pH. Your mash pH is more likely to be too high than too low.