Help with understanding set analysis and using the result in a table

[treborscottnam]

Hello,

I have a table that for each month shows what the ending of the month headcount is. It uses this expression:
Month Headcount = Sum( {<IsLastDay={"Y"}>} Active)

In order to do a calculation (turnover) I need to also get the first day of the year headcount and use that number in each row of the table.
I can get the first day of the month, for the first month number with this expression:
First Day of Year =Sum( {<IsFirstDay={"Y"}, Month = {"1"}>} Active)

My question is:  How can I repeat that first day of the month in the table or get that result in each row's calculation. The final result would be using this expression: =YTD Terminations / (First Day of Year + Month Headcount)/2

Where I am stuck is that it only shows up in the first month. Here's what the table looks like now:

Year	Month	Headcount	Terminations	First Day of Year
2015	1	5000	635	4950
2015	2	5084	457
2015	3	5187	565
2015	4	4922	240
2015	5	5352	502
2015	6	5250	680
2015	7	5270	275
2015	8	5218	196
2015	9	4800	544
2015	10	5875	646
2015	11	5466	420
2015	12	4927	179
2016	1	5855	360	5560
2016	2	5887	310
2016	3	5464	420
2016	4	5984	433
2016	5	5464	356
2016	6	5123	411
2016	7	5649	345
2016	8	5345	353
2016	9	5935	304
2016	10	5140	313
2016	11	5456	383
2016	12	5949	327

I want it to look like is this:

Year	Month	Headcount	Terminations	First Day of Year
2015	1	5000	635	4950
2015	2	5084	457	4950
2015	3	5187	565	4950
2015	4	4922	240	4950
2015	5	5352	502	4950
2015	6	5250	680	4950
2015	7	5270	275	4950
2015	8	5218	196	4950
2015	9	4800	544	4950
2015	10	5875	646	4950
2015	11	5466	420	4950
2015	12	4927	179	4950
2016	1	5855	360	5560
2016	2	5887	310	5560
2016	3	5464	420	5560
2016	4	5984	433	5560
2016	5	5464	356	5560
2016	6	5123	411	5560
2016	7	5649	345	5560
2016	8	5345	353	5560
2016	9	5935	304	5560
2016	10	5140	313	5560
2016	11	5456	383	5560
2016	12	5949	327	5560

It doesn't have to actually look like that, I just need to get the First Day of Year number. That is so, as I said above, I can use this calculation:
=YTD Terminations / (First Day of Year + Month Headcount)/2

So the end result would be this:

Year	Month	Headcount	Terminations	First Day of Year	Turnover
2015	1	5000	635	4950	3.2%
2015	2	5084	457	4950	5.4%
2015	3	5187	565	4950	8.2%
2015	4	4922	240	4950	9.6%
2015	5	5352	502	4950	11.6%
2015	6	5250	680	4950	15.1%
2015	7	5270	275	4950	16.4%
2015	8	5218	196	4950	17.5%
2015	9	4800	544	4950	21.0%
2015	10	5875	646	4950	21.9%
2015	11	5466	420	4950	24.8%
2015	12	4927	179	4950	27.0%
2016	1	5855	360	5560	1.6%
2016	2	5887	310	5560	2.9%
2016	3	5464	420	5560	4.9%
2016	4	5984	433	5560	6.6%
2016	5	5464	356	5560	8.5%
2016	6	5123	411	5560	10.7%
2016	7	5649	345	5560	11.8%
2016	8	5345	353	5560	13.7%
2016	9	5935	304	5560	14.3%
2016	10	5140	313	5560	16.8%
2016	11	5456	383	5560	18.1%
2016	12	5949	327	5560	18.7%

I had tried some different approaches such as using variables and trying out using an equal sign in the expression, and sometime not. I'm still a bit fuzzy on when the expression calculates based on if there is an equal sign or not in the expression.

Thank you for the help!