The only way to determine MHR is to do a test and use the highest you can see after a hard 30' session or intervals of 4'/1'R stepped down to failure (say 6/7). There are no formulae that have a sensible role to play in calculations of MHR.
Chest strap or upper arm is needed - watch is not useful for rowing.
Your MHR is a genetic fact of life that doesn't change with fitness levels. Your RHR drops with as fitness improves.
Most training should be slow enough to keep your heart rate below 80% of your heart rate reserve (there are lots of threads here to explain all that)
There is nothing IMO wrong with approaching your real MHR on hard AN pieces especially as your fitness improves. Have a look at the Free spirits charts linked above.
JEPonline Journal of Exercise Physiology online
Volume 5 Number 2 May 2002
Commentary
THE SURPRISING HISTORY OF THE “HRmax=220-age” EQUATION
ROBERT A. ROBERGS AND ROBERTO LANDWEHR
Exercise Physiology Laboratories, The University of New Mexico, Albuquerque, NM
ABSTRACT
THE SURPRISING HISTORY OF THE “HRmax=220-age” EQUATION. Robert A. Robergs, Roberto
Landwehr. JEPonline. 2002;5(2):1-10. The estimation of maximal heart rate (HRmax) has been a feature of
exercise physiology and related applied sciences since the late 1930’s. The estimation of HRmax has been
largely based on the formula; HRmax=220-age. This equation is often presented in textbooks without
explanation or citation to original research. In addition, the formula and related concepts are included in most
certification exams within sports medicine, exercise physiology, and fitness. Despite the acceptance of this
formula, research spanning more than two decades reveals the large error inherent in the estimation of HRmax
(Sxy=7-11 b/min). Ironically, inquiry into the history of this formula reveals that it was not developed from
original research, but resulted from observation based on data from approximately 11 references consisting of
published research or unpublished scientific compilations. Consequently, the formula HRmax=220-age has no
scientific merit for use in exercise physiology and related fields. A brief review of alternate HRmax prediction
formula reveals that the majority of age-based univariate prediction equations also have large prediction errors
(>10 b/min). Clearly, more research of HRmax needs to be done using a multivariate model, and equations may
need to be developed that are population (fitness, health status, age, exercise mode) specific.