Really need help understanding construction practical

[What?]

So I start a game as Imperial Japan and line up 3 factories for construction
The day before they are about to finish i saved the game and looked at the values for a new factory, the mouse hover tooltip tells me that if i were to lay down a new factory that day, it would have 4% of the cost and time to construct saved.

Now, the day after the three new factories are completed, i look at my values and the tooltip shows me that the savings shrank to 2.7%...

Wouldn't the completion of 3 new factories mean my construction practicals grow instead of actually decay?
This was a week into the month, and really i dont expect three factories completed to have their construction practical erased by one month's decay.

In the research section my practical went up 2 points from 5.2 to 7.2, which one would assume means something somewhere is growing and not shrinking

What's going on here? Are these values accurate?

*edit*
thanks for the replies below, i guess i didn't look closely at that figure and didnt notice the - signs

 

[Kovax]

Not sure why the tooltip is showing what it does, but here's another tip:

Rather than have 3 factories finish on the same day, and have the Practical you get from building the first be wasted on the others, shift the others to the bottom of the Production que, so something else gets worked on for a while. That way, the ones at the bottom fall a few IC/days behind. When the first one finishes, the others will generally move up in completion date by a week or more, saving all of that production time. Ideally, you want them spread out so one completes and provides just enough boost so the second finishes the following day, and that second one causes the third to complete the next day, etc., but that's far too complex to calculate, so I just "guesstimate". With a production run of just 3 IC, you're probably only going to save a couple of weeks of production time on the last factory, but that's still a couple of weeks of several IC which could have been put toward something else. For GER, US, or SU building 10+ at a shot, the savings could amount to several "free" divisions, or enough to build a couple more IC at no extra cost.

 

[Charles Reeps]

So I start a game as Imperial Japan and line up 3 factories for construction
The day before they are about to finish i saved the game and looked at the values for a new factory, the mouse hover tooltip tells me that if i were to lay down a new factory that day, it would have 4% of the cost and time to construct saved.

Now, the day after the three new factories are completed, i look at my values and the tooltip shows me that the savings shrank to 2.7%...

Wouldn't the completion of 3 new factories mean my construction practicals grow instead of actually decay?
This was a week into the month, and really i dont expect three factories completed to have their construction practical erased by one month's decay.

In the research section my practical went up 2 points from 5.2 to 7.2, which one would assume means something somewhere is growing and not shrinking

What's going on here? Are these values accurate?

When the Construction Practical was at 5.2 you had a MINUS 4% impact on cost and time. The three new IC properly raised Construction Practical by 2 points, resulting in a POSITIVE 2.7% impact in cost and time. In the former you would then multiply the base cost and time of IC construction by 1.04 (an increase in cost and time); in the latter you would multiply base cost and time of IC construction by .973 (a decrease in cost and time). You have to pay attention to actual cost and time savings to truly gauge what is going on.

 

[KyrionMyrthar]

I listen to what Charles says. Math isn't my strong suit in this game, and he's very clear about how it all works.

 

[athunder7]

A week usually works on boosting complete times. You gain it a start of construction too by leaving IC in supplies 7-8 days for second factory the 1-2 for factories after that. My best gain was in BB and BC construction finishing BC, BC,BB, BC, BB, BC, within a month but start dates were 1 year off. You can try 5 airfield trick for supper jump in IC factories building.

 

[21oliver]

Perhas your confused. Higher practicals are good. If your practicals go UP, that means construction time goes down.