Dividing the cost per share into the dollar amount to get the shares will never quite work out . . . especially when dealing with large figures. In the TSP your shares always stay the same from the time of purchase (or exchange) to the next purchase (or exchange).
You will find rounding errors both ways when making exchanges, but it is best to attribute these to the price. I've been surprised only slightly -- both up and down -- in regard to price. Again your shares stay the same.
If you are tracking in Quicken, I especially recommend you track shares and the total cost of the transaction . . . and let the price fluctuate the amount of the rounding error.
I'll be interested in what you find if you look at several transactions. It has been my experience that the price I received was sometimes very slightly up or down, but in general the calculation has been on target. . . BUT it always pays to keep on eye on folks managing your funds.
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Here is a real-world example from my funds. In order to keep my options more flexible, I make several 1% exchanges from time to time. (If the market changes direction, I can add to each of those so long as it is under the "1% rule" [see that discussion elsewhere] even if I have used both non-G exchanges).
So . . . yesterday, I made several 1% exchanges.
Even though each was 1%, the actual percentage (therefore the cost of the transaction) varies. As it turns out, it is difficult to get the exact amounts.
Below are two of the 1% exchanges. The actual percentages carried to 7 decimals shows the rounding effect:
0.9999967%
1.0000029%
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Just for grins, I totalled all the percentages out to 9 decimals. Even though it looks like (because rounding error), based the percentages, I ended up a winner:
100.000000121%
The actual dollar figure was correct to the penny--before and after.
