I actually don’t do that routinely for my Thrift Savings Plan. I have a reasonable sense of how things have performed in my head and am not concerned enough to want to know precisely.
For the purposes of the last update, I went to TSPcenter.com and checked the TSP S Fund return from 1/1 through the date of my transfer. Then I checked the S Fund and I Fund returns for that date to the date of the update and multiplied those by 85% and 15% respectively. Then added those all up to get my return for the year to date.
I’m sure there is a more elegant way to do that, but it worked for my purposes.
If you log into your account, on the left hand side under Account Information, select the link for Activity Summary. It usually defaults to the current quarter, but you can put 01/01/2015 or any date within the last 2 years and it will calculate your actual gain or loss for that period. The percentage gain/loss can be calculated by dividing the gain/loss by (beginning balance + half of your contributions), which will give you a close approximation assuming your contributions are equal throughout the year.
Under your account summary, there is also a link: “View your 12-Month Personal Investment Performance (PIP)” that is calculated each month by 3rd business day.