We analyze consequences of the approximation \mu_\pi^2 -\mu_G^2 << \mu_\pi^2
(a `BPS' limit) for B and D mesons. It is shown that neglecting perturbative
effects many power corrections would vanish to all orders in 1/m_Q, in
particular those violating heavy flavor symmetry. Among them are corrections to
B->D formfactors. A number of relations receive corrections only to the second
order expanding around the BPS limit to any order in 1/m_Q, including both f_+
and f_- at zero recoil. This allows an accurate evaluation of {\cal F}_+ for
B->D l\nu. Its perturbative renormalization is computed analytically in the
required Wilsonian scheme, yielding the dominant 3% enhancement.Comment: 13 pages, LaTeX, two figure
