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Use the UFactorTimesAreaAndDesignWaterFlowRate method and autosize the UA and water flow rate. Then use the autosized value for these two fields. That should be all that's necessary.

From the 5ZoneAirCooled example file:

Coil:Heating:Water,
SPACE1-1 Zone Coil,      !- Name
ReheatCoilAvailSched,    !- Availability Schedule Name
autosize,                !- U-Factor Times Area Value {W/K}
autosize,                !- Maximum Water Flow Rate {m3/s}
SPACE1-1 Zone Coil Water In Node,  !- Water Inlet Node Name
SPACE1-1 Zone Coil Water Out Node,  !- Water Outlet Node Name
SPACE1-1 Zone Coil Air In Node,  !- Air Inlet Node Name
SPACE1-1 In Node,        !- Air Outlet Node Name
UFactorTimesAreaAndDesignWaterFlowRate,  !- Performance Input Method

The eio output for this coil:

Component Sizing Information, Coil:Heating:Water, SPACE1-1 ZONE COIL, Design Size Rated Capacity [W], 4624.09792
Component Sizing Information, Coil:Heating:Water, SPACE1-1 ZONE COIL, Design Size Maximum Water Flow Rate [m3/s], 1.02164E-004
Component Sizing Information, Coil:Heating:Water, SPACE1-1 ZONE COIL, Design Size U-Factor Times Area Value [W/K], 113.24397
! <Water Heating Coil Capacity Information>,Component Type,Name,Nominal Total Capacity {W}
Water Heating Coil Capacity Information,Coil:Heating:Water,SPACE1-1 ZONE COIL,3748.67

Revised SPACE1-1 Zone Coil object with hard values - only the flow rate and UA (Rated Capacity input is ignored for this performance method):

Coil:Heating:Water,
SPACE1-1 Zone Coil,      !- Name
ReheatCoilAvailSched,    !- Availability Schedule Name
113.24397,                !- U-Factor Times Area Value {W/K}
1.02164E-004,                !- Maximum Water Flow Rate {m3/s}

The eio output for the hard-sized coil:

! <Water Heating Coil Capacity Information>,Component Type,Name,Nominal Total Capacity {W}
Water Heating Coil Capacity Information,Coil:Heating:Water,SPACE1-1 ZONE COIL,3748.67

The same nominal capacity as the autosized case. This should be reliable for Coil:Heating:Water. And the simulation results are the same for these two runs.