Plan Date Calculation on Performance Based PM Scheduling

Dear EAM Expert,

Presently, we have implementing Preventive Maintenance using SAP-PM.

The Preventive Maintenance Scheduling is using Performance Based (Hours) of Equipment.

But, now we are confusing about "Plan Date" that proposed by system when reviewing Maintenance Scheduling by IP10

The proposed date seems like too far before it should be.

For Example:

There are Preventive Maintenance for 2000 Hours.

Current Counter Reading is 1339 Hour

But, when we Scheduling Maintenance using IP10, It will call the schedule for Maintenance 2000 Hours.

Based on manual calculation, since Scheduling Parameter of this Maintenance is using 88% Call Horizon,

Preventive Maintenance for 2000 Hours should be called at Counter Reading 1760.

We try to analyse the date calculation by get "Call Algorithm",

But, actually we got more confusing.

Please, can anybody help me for explanation about Scheduling Call Date algorithm?

Thanks

Adi